Rebar Calculator - Bars, Lap Splices, Weight & Cost

How to use it

1. Pick what you are reinforcing. Slab grid is the default and covers a garage floor, a shed pad, a patio or a driveway. Footing handles a continuous strip footing with corner bars. Mesh vs rebar prices welded wire against a bar grid on the same slab. Weight only turns a list of bars into pounds and tons.
2. Enter the slab in feet and inches, with the inches in their own box. Two separate boxes rather than one decimal field is deliberate: 12 ft 6 in is not 12.6 ft, and a single field is where that mistake happens. The fields accept a comma or a period as the decimal mark, so a browser set to French or Spanish shows the same number you typed.
3. Add more bays if the pour is not one rectangle. Up to four areas, each with its own name and a count for repeats, and every offcut from every area goes into one shared cut plan, which is exactly what happens on site when the pile of sticks is in the driveway.
4. Choose the bar size. #4 is the residential default and the size the seeded prices are checked against. #3 suits a light patio, #5 a heavy driveway or a thicker footing. Sizes above #6 are in the printable chart but not the picker, because at that point you are reading a drawing rather than a calculator.
5. Set the spacing, independently in each direction if you want it. Presets are 12, 16, 18 and 24 in on centre and the box is editable. Bars running lengthwise are spaced across the width, bars running crosswise are spaced along the length, and the drawing labels both so there is no guessing which number moved.
6. Set the edge clearance. Three inches is the working default, matching the cover concrete wants where it is cast straight against earth. The grid span is the slab dimension minus twice that figure, which is the single most common place a takeoff goes wrong: people count bars across the slab instead of across the span.
7. Say what stock you can get and how long the lap is. Twenty foot sticks are the cheapest per foot and will not fit in any pickup bed. Ten foot sticks fit the truck and cost more, because every run needs more splices. The lap defaults to 40 bar diameters, which is 20 in on a #4, and it is an editable field because your inspector's number wins over ours.
8. Set the chairs and the tie wire. Chairs every 4 ft is the usual guidance and they are what keeps the steel in the middle of the slab instead of lying on the dirt doing nothing. Ties default to every other intersection, which is normal practice for a flat grid, and the tool converts that into rolls.
9. Read the cut list. It is the part no other calculator prints: how many sticks go in whole, how many get cut, what the cuts measure and what the drop is on each one. If the drop is over 15 percent of what you are buying the page says so, because that is usually a sign the yard should cut to length instead.
10. Edit the cost ledger to your own quotes. Price per stick, the bulk price and the quantity that unlocks it, chairs each, tie wire per roll, mesh per sheet and an optional delivery fee. When the count lands just under the bulk break the page shows you what buying two extra sticks would actually cost, which on the default job is a $75.50 saving for more steel.

At a glance

• 24 by 24 ft garage slab, #4 at 16 in on centre, 20 ft sticks: Span 23 ft 6 in each way, 18 bars each way, 36 runs. Every run needs one splice, so 846 ft of run becomes 906 ft of steel in the pour, which is 605.2 lb. The cut plan is 36 whole sticks plus 12 donors cut three ways at 5 ft 2 in: 48 sticks. The shortcut says 43. Add 49 chairs and 162 ties on one roll and the order is $689.05, or $584.50 if you take 50 sticks at the bulk price.
• 20 by 20 ft slab, the size where the shortcut happens to be right: Span 19 ft 6 in, 15 bars each way, 30 runs, and every one of them fits inside a 20 ft stick with 6 in to spare. No splices, no lap steel, 585 ft of bar at 390.8 lb, 30 sticks. Divide 585 by 20 and you also get 30. The agreement is a coincidence of the size, and it ends the moment a run is 6 in longer.
• 80 ft of footing for a 16 by 24 ft addition, two #4 bars: Each bar line takes five pieces: four whole sticks reach 75 ft after three laps, and a 6 ft 8 in closer finishes it. Two lines plus eight corner bars at 4 ft each is 205 ft 4 in of steel, 137.2 lb, and it packs into 11 sticks. The shortcut says 10. Forty two supports, 32 ties, $177.15.
• 12 by 16 ft shed slab, mesh against bar: Six sheets of 5 by 10 ft mesh laid out with a 6 in lap, $78.00 of mesh in an $87.00 order. The same slab in #4 at 16 in is 21 sticks and $304.75, because a 15 ft 6 in cut and an 11 ft 6 in cut each burn a whole 20 ft stick. Mesh is the economical crack control for a light slab. Bar earns its money once a vehicle parks on it.

How the math works

Everything on this page comes off one chain, and the whole chain is printed in the result so you can check it on paper. It starts with the span, which is where most takeoffs go wrong. The grid does not run to the edge of the slab: it stops short of it by the edge clearance, on both sides, so the span is the slab dimension minus twice that figure. A 24 ft slab with 3 in of clearance has a 23 ft 6 in span, not 24 ft, and counting bars across 24 ft instead of 23 ft 6 in is a whole extra bar in each direction. Bar count is then the perpendicular span divided by the spacing, rounded down, plus one, because a grid of n gaps has n plus 1 bars. At 16 in on centre across a 23 ft 6 in span that is floor of 282 divided by 16, which is 17, plus 1, which is 18. Eighteen bars each way on a square slab is 36 runs of 23 ft 6 in. Now the part the shortcut skips. Each of those runs is longer than a 20 ft stick, so it is spliced, and a splice is an overlap: two bars lie side by side for the lap length and get tied together, so the steel that ends up in the concrete is longer than the run it covers. The lap defaults to 40 bar diameters, which on a #4 at half an inch is 20 in, or 1 ft 8 in. The piece count for a run is one piece if the run fits a stick, and otherwise one plus the ceiling of the run minus the stock length, divided by the stock length minus the lap, because after the first stick every additional piece only advances you by what it adds beyond its own lap. On a 23 ft 6 in run that is 1 plus the ceiling of 3.5 divided by 18.333, which is 2 pieces and 1 splice. Total steel in that run is 23.5 plus 1.667, which is 25 ft 2 in. Across 36 runs, 846 ft of run becomes 906 ft of bar, and at the ASTM A615 nominal weight of 0.668 lb per foot for a #4 that is 605.2 lb sitting in the slab. Then the cut plan, which is the engine and the reason the answer differs from everyone else's. Those 2 pieces are not two random lengths: the first is a whole 20 ft stick and the second is 5 ft 2 in, because 25 ft 2 in minus 20 ft is 5 ft 2 in. So the job needs 36 whole sticks and 36 tails of 5 ft 2 in. A 20 ft stick yields three of those tails with 4 ft 6 in of drop, so the 36 tails come out of 12 donor sticks. Thirty six plus twelve is 48 sticks. That is a one dimensional cutting stock problem and the tool solves it the way a person solves it: take the stick in your hand, cut the longest piece that still fits, repeat, and when nothing else fits start a new stick and throw the tail on the scrap pile. Every offcut from every area goes into the same pool, so a second bay's short bars can come out of a first bay's drop. Compare that with the shortcut everyone else prints, 846 divided by 20 is 42.3 so call it 43, and you are five sticks short before you start, because the shortcut assumes bar can be joined end to end at no cost. The rest of the order is arithmetic on the same grid. Chairs are laid on their own grid, so the count is the ceiling of the span divided by the chair spacing, plus one, in each direction, multiplied together: 6 plus 1 by 6 plus 1 is 49 at 4 ft spacing. Ties are a fraction of the intersections, and the intersections are just bars one way times bars the other, so 18 by 18 is 324, tied every other one is 162, which is well inside one 3.5 lb roll of 16 gauge wire at roughly 500 ties a roll. Weight is length times the nominal pounds per foot, computed twice: once on the steel in the pour and once on the steel you buy, because the difference between them is the drop, and on this job that is 54 ft and 36 lb of scrap you paid for. A footing runs the same machinery on a single long run instead of a grid: the continuous bar lines are spliced the same way, and corner bars, two per corner with 2 ft legs, are added as separate 4 ft pieces that get packed into the same sticks. Mesh is laid out as a real grid rather than by area division, because sheets lap where they meet and not at the slab edge, so n sheets in a line cover n times the sheet size minus n minus 1 laps, and inverting that is what stops the count over-buying on a slab that is close to a whole number of sheets. Money is nothing but each quantity times a price you set, rounded to the cent row by row so the column always adds to the total printed under it, plus one honest extra: when your count lands just under the quantity that unlocks a bulk price, the page prices both and tells you which is cheaper, which on the default job means 50 sticks at $11.69 costs $584.50 against 48 at $13.75 costing $660.00.

1. 1. Take the span, not the slab

   The grid stops short of the slab edge by the edge clearance, on both sides, so the span is the dimension minus twice that figure. A 24 ft slab at 3 in clearance has a 23 ft 6 in span. Getting this wrong in the obvious direction, counting across the full 24 ft, adds a bar in each direction and about 47 ft of steel you will not place. Getting it wrong the other way, subtracting the clearance once instead of twice, is the same mistake in miniature. Three inches is the working default because concrete cast straight against earth wants that much cover under the steel, and it is an editable field because a formed edge is normally allowed less. Confirm the number with your building department before you order.

2. 2. Count bars as gaps plus one

   Bars per direction is the perpendicular span divided by the spacing, rounded down, plus one, because a run of n gaps is fenced by n plus 1 bars. Across a 23 ft 6 in span at 16 in on centre: 282 divided by 16 is 17.6, floor 17, plus 1 is 18 bars. Forget the plus one and you are a bar short in each direction on every job you ever do. Spacing is set independently for each direction here, because a slab that is long and narrow often wants tighter steel across the short way, and the two numbers are not required to match.

3. 3. Splice every run that outgrows a stick

   A run longer than the stock length has to be spliced, and a splice is an overlap rather than a joint: two bars lie together for the lap length and are tied. Pieces for a run is 1 if the run fits a stick, otherwise 1 plus the ceiling of the run minus the stock, over the stock minus the lap. On a 23 ft 6 in run with 20 ft sticks and a 20 in lap that is 2 pieces and 1 splice, and the steel in that run measures 25 ft 2 in rather than 23 ft 6 in. Across 36 runs the laps alone are 60 ft of bar, 40 lb of steel that the shortcut never counts and you still have to pay for.

4. 4. Solve the offcuts, do not average them

   Each spliced run is a whole 20 ft stick plus a 5 ft 2 in tail. Three tails come out of one donor stick with 4 ft 6 in of drop, so 36 tails need 12 donors and the order is 36 plus 12, which is 48 sticks. That is a one dimensional cutting stock problem, and the tool solves it by doing what a person does: cut the longest piece that still fits off the stick in your hand, and when nothing fits start a new one. It is not a waste percentage. A waste percentage on this job would have to be 13.5 percent to reach the same answer, and on the 12 by 16 ft shed slab it would have to be 51 percent, which is why nobody can pick one that works.

5. 5. Print the shortcut beside the answer

   Total run length divided by stock length is what the calculators ranking above this page compute: 846 divided by 20 is 42.3, so 43 sticks. It is not a rounding difference from the real answer, it is a different question. It asks how many sticks of steel weigh as much as this grid, and the answer to that is genuinely 43. What you have to buy is 48, because bar cannot be joined end to end. Both numbers are on screen at once here, with the gap between them named, so you can see which one you have been using.

6. 6. Count the chairs and the wire, they are on the same receipt

   Steel lying on the dirt does nothing at all: the whole point of reinforcement is that it sits where the concrete is in tension, which for a slab on grade is the middle to lower third. Chairs hold it there. They go on their own grid, so the count is the ceiling of each span over the chair spacing, plus one, multiplied: 49 chairs at 4 ft on the reference slab. Ties are a share of the intersections, and 18 by 18 bars gives 324 intersections, tied every other one is 162, comfortably one roll of 16 gauge wire. Neither line is large money and both are the difference between a grid that stays put during the pour and one that does not.

7. 7. Weigh it twice, once in the pour and once in the truck

   In-place weight is the steel that ends up in the concrete, laps included: 906 ft at 0.668 lb per foot is 605.2 lb. Bought weight is what you carry home, which is 48 sticks at 20 ft, so 960 ft and 641.3 lb. The 54 ft and 36 lb between them is drop, and it is real money you already spent. The bought figure is also the one to check against your truck: the payload sticker inside the driver's door, not the brochure number. And no matter what the sticker says, a 20 ft stick overhangs every pickup bed made, so the practical options are delivery, a cutting service, or 10 ft sticks and more splices.

8. 8. Price it as a ledger you can argue with

   Every price on this page is an editable field with the date it was checked printed beside it, and the total is nothing but quantity times price, rounded to the cent row by row so the column adds to the figure under it. The one piece of genuine arithmetic in the money section is the bulk break: when your count lands just under the quantity that unlocks a lower price, buying more steel can cost less. Forty eight sticks at $13.75 is $660.00. Fifty sticks at $11.69 is $584.50. That is $75.50 saved and two spare sticks, and it is exactly the kind of thing a takeoff sheet is for.

Worked scenarios, start to finish

Real jobs with the numbers carried all the way through, so you can see how the estimate is built and check your own figures against them.

The judgement calls a pro makes

Decisions the tool cannot make for you, and how someone who does this for a living thinks them through.

Mesh, bar, or fibre

None of the three do the same job. Welded wire mesh is crack control: it does not stop a slab cracking, it holds the crack tight after it forms, and on a 4 in patio or shed pad it is the economical answer at well under half the cost of a bar grid. Fibre mixed into the concrete controls the fine shrinkage cracking that happens in the first day and replaces nothing structural. Rebar is the one that carries load across a crack once it exists, and it earns its cost on driveways, on slabs thicker than 4 in, on anything a vehicle sits on and on anything built over questionable soil. Plenty of small slabs get all three, and plenty get mesh alone and last thirty years.

#3, #4 or #5

For flat residential work the honest range is narrow. #3 at 3/8 in is enough for a light patio or a walkway and is the size people reach for when they want something better than mesh without the weight. #4 at 1/2 in is the residential default and what almost every prescriptive detail assumes: it is stiff enough to stay where you put it and light enough to carry. #5 at 5/8 in belongs in a heavy driveway, a thicker footing or a retaining stem, and it is 56 percent heavier per foot than a #4, which you feel in both the price and the handling. Going up a size is rarely the right fix for a slab you are worried about. Tightening the spacing or thickening the concrete usually is.

12, 16 or 18 in on centre

On the reference 24 by 24 ft slab the three spacings are 64, 48 and 43 sticks, which is $880, $660 and $591 of steel. Sixteen inches is the common default for a residential slab and the one most prescriptive details assume. Twelve inches is what you use under a heavy vehicle or where a local detail calls for it. Eighteen and 24 in show up on light patios where the steel is really doing crack control. The interesting part of those numbers is that going from 16 to 18 in only saves five sticks, because the bar count drops but the lap and offcut arithmetic barely moves. Tightening the grid is cheaper than it looks, and loosening it saves less than it looks.

Chairs, or pulling the steel up during the pour

Steel lying on the ground is doing nothing, because it is not where the concrete is in tension. The two ways to fix that are chairs, which cost about $22 on a 24 by 24 slab, and hooking the mesh or bar up with a rake as the concrete goes in, which costs nothing and works about as well as it sounds. On a small pour with two people who have done it before, pulling up is survivable. On anything bigger, or on any pour where you are also screeding, chairs are the cheapest insurance on the whole job. Concrete brick or dobie blocks do the same thing for less if you have them.

20 ft sticks or 10 ft sticks

Twenty foot sticks are cheaper per foot and need fewer splices: the reference slab is 48 sticks and $660.00 of steel. The same slab in 10 ft sticks needs 72 splices instead of 36, 108 sticks, and $810.00, so the short stock costs $150 more and puts 60 more feet of lap steel in the slab. What buys that back is that 10 ft sticks fit in a pickup bed and 20 ft sticks do not fit in any pickup bed, so the real comparison is $150 against a delivery fee or a trailer rental. If the yard will cut 20 ft stock to your list, that is usually the cheapest of the three.

Bending your own bar or buying it bent

Corner bars, hooks and stirrups can be bent on site with a hand bender, and for eight corner bars on a small footing that is an hour of work and no money. Grade 60 bar does not bend kindly by hand above #4, and a cold bend that is too tight can crack the bar, which is why the bend diameter is specified rather than left to feel. If the job needs more than a couple of dozen bends, or any bend in #5 and up, price a fabricator: they will cut and bend to a list and deliver it tagged, and on a footing job the labour saved usually beats the premium.

When to stop calculating and call an engineer

This page covers slabs on grade and continuous footings, which are the two places where residential practice is prescriptive enough that a calculator can help. Everything else on a house is not: a suspended slab, a beam, a column, a retaining wall holding back soil, a footing carrying a point load from a post or a beam pocket, anything on expansive clay, and anything where the drawing says the bar size and spacing rather than leaving it to you. That needs a design from an engineer, not a calculator. The tell is simple: if the steel is what is holding the thing up rather than holding a crack together, stop.

Common mistakes, and the fix

• × Dividing total linear feet by stick length. It is the shortcut every calculator on the first page prints, and it under-buys any slab whose runs are longer than a stick.

  ✓ Count the pieces, not the feet. A run longer than a stick needs 1 plus the ceiling of run minus stock over stock minus lap, and each splice adds the lap to the steel you place. On the reference slab that is 43 against 48, a five stick miss.

• × Counting bars across the slab dimension instead of across the grid span.

  ✓ Subtract the edge clearance twice, once for each side, before you divide by the spacing. A 24 ft slab at 3 in clearance has a 23 ft 6 in span. It is one bar per direction and about 47 ft of steel, and it is the single most common error in a rebar takeoff.

• × Laying the grid straight on the dirt or on the vapour barrier, then pouring over it.

  ✓ Chair it. Steel at the bottom of a slab on grade is close to useless, because it is not in the part of the section that goes into tension. Chairs every 4 ft cost about $22 on a two car garage slab, and dobie blocks cost less if you have them.

• × Cutting the laps short because the tail looked long enough.

  ✓ The lap is what makes two bars behave as one, so a short lap is a hinge in the middle of the slab. Forty bar diameters is the common residential rule of thumb, which is 20 in on a #4, and your local building department's number wins. If the closing piece will not reach, use a longer piece rather than a shorter lap.

• × Ordering 20 ft sticks for a job where the truck is a half ton pickup.

  ✓ A 20 ft stick overhangs every pickup bed made, and no amount of red flag makes that legal in most states. Price the delivery, ask whether the yard will cut to your list, or switch to 10 ft stock and accept the extra splices. The tool prices all three.

• × Assuming a waste percentage covers the offcuts on a slab with short runs.

  ✓ It cannot, because the offcut depends on how the run length divides into the stock, not on how much steel there is. The reference slab needs 13.5 percent to break even and the 12 by 16 ft shed slab needs 51 percent. Solve the cut plan instead, which is what the cut list on this page is.

Material and unit specifics

The size number is eighths of an inch, up to #8

A #4 bar is 4/8 in, which is 1/2 in. A #3 is 3/8 in and a #6 is 3/4 in. The rule is exact from #3 to #8 and then it stops: #9, #10 and #11 are sized so their cross sectional area matches the old square bar sizes of 1.00, 1.27 and 1.56 sq in, which makes their diameters 1.128, 1.270 and 1.410 in rather than the 1.125, 1.250 and 1.375 the eighths rule would predict. Nobody on the first page of this search mentions the exception, and it matters the moment you compute a lap length from the diameter.

Weight per foot, and where it comes from

The ASTM A615 nominal weights are #3 at 0.376, #4 at 0.668, #5 at 1.043, #6 at 1.502, #7 at 2.044, #8 at 2.670, #9 at 3.400, #10 at 4.303 and #11 at 5.313 lb per foot. They are not measured, they are derived: steel weighs about 490 lb per cubic foot, so a bar of area A square inches weighs A over 144 times 490 pounds per foot. A #4 is 1/2 in across, so its true area is 0.1963 sq in, and 0.1963 over 144 times 490 is 0.668 lb per foot, exactly the published figure. The 0.20 sq in you see in tables is that same area rounded for convenience, which is why running the arithmetic on 0.20 gives a slightly high 0.681. A 20 ft #4 stick is 13.4 lb, so a 48 stick order is 641 lb of steel.

Grade 40 and Grade 60, and how to read the bar

Grade 60 is the common stock now and Grade 40 survives mostly in the smaller sizes. The bar tells you which it is: rolled into the surface you will find the producer's mill mark, then the size number, then a letter for the steel type, then the grade, shown either as the number 60 or as a single longitudinal line rolled through the deformations. Grade refers to yield strength in thousands of pounds per square inch, so Grade 60 yields at 60,000 psi. It matters here only because a prescriptive detail assumes a grade, so if a drawing says Grade 60 and the yard hands you Grade 40, that is a question for the person who wrote the drawing.

Black, epoxy coated and galvanized

Plain black bar is what almost every residential slab and footing uses and what the prices on this page assume. Epoxy coated bar, the green stuff, is for bridge decks, parking structures and anywhere de-icing salt reaches the steel, and it costs substantially more and has to be handled carefully because a nicked coating is worse than no coating at the nick. Galvanized sits between them. For a garage floor or a shed pad, black bar with proper cover is the answer, and cover is what actually protects steel in concrete: the concrete's own alkalinity passivates the surface as long as the bar is not too close to the outside.

Lap length, and why 40 diameters is a rule of thumb

Forty bar diameters is the number the trades use, giving 15 in on a #3, 20 in on a #4, 25 in on a #5 and 30 in on a #6. It is a simplification of a real calculation that depends on concrete strength, bar coating, bar spacing, cover, whether the bar is at the top of a deep pour and how many bars are spliced in the same place, and the real answer can be shorter or considerably longer than 40 diameters. Treat the field here as a default to overwrite with whatever your building department or your drawing says, not as a specification.

Chairs, dobies and tie wire

Chairs are the plastic or wire supports that hold the grid at the right height. Individual plastic chairs run about $0.45 each and a 4 ft grid is the usual guidance, so a two car garage slab takes about 49 of them. Concrete dobie blocks with a wire loop do the same job, cost less, and will not punch through a vapour barrier. Ties are 16 gauge annealed wire, about 500 ties to a 3.5 lb roll, twisted with a hook or a battery tool at every other intersection on a flat grid. The ties are not structural, they exist to keep the grid where you put it while the concrete goes in and people walk on it.

Welded wire mesh sizes

The common residential mesh is 6x6 W1.4/W1.4, meaning a 6 by 6 in grid of wire with 0.014 sq in of steel each way, the product formerly called 10 gauge. Concrete supply yards stock it as 5 by 10 ft sheets at about 10.5 lb each; big box stores usually carry a smaller 3.5 by 7 ft remesh sheet, and both sizes are editable in the tool. Rolls come 5 ft by 150 ft and are cheaper per square foot, at the cost of fighting a sheet that badly wants to roll back up. Sheets lap one full square where they meet, which is 6 in, and that lap is exactly what an area division ignores.

Rebar caps are not optional

Vertical bar sticking out of a footing or a slab is an impalement hazard, and the plastic mushroom caps that cover it cost pennies. OSHA requires protection on job sites and the physics does not care whether you are being paid. If any bar on your job points up and anyone is going to be working above or around it, cap it or bend it over before you walk away for the night.

Reading your result

Read the two headline numbers together, because the gap between them is the point of the page. The large figure is what you buy: whole sticks plus the sticks that get cut into the tails your runs need, solved as a real cut plan. The muted figure beside it is total linear feet divided by stick length, which is what the calculators ranking above this one print, and it is short by however much lap steel your grid needs plus whatever the offcuts cannot supply. When the two agree, as they do on a 20 by 20 ft slab, it is because no run exceeds a stick, and the page says so. Under those, the cut list is the practical output: hand it to whoever is cutting and they know how many sticks go in whole, how many get cut and what the cuts measure. If the drop line is over about 15 percent of what you are buying, the layout is fighting the stock length, and the fix is usually to ask the yard to cut to your list rather than to buy more steel. The weight figures are for the truck, and the in-place figure is the one to quote if anyone asks how much steel is in the slab. The cost ledger is a planning ledger and not a quote: it is your quantities against prices you can edit, so use it to sanity check a supplier's number rather than to replace it, and remember the bulk line is real money, because two spare sticks are frequently cheaper than the twelve you already had in the cart. And every structural figure here, the spacing, the bar size, the lap, the cover, the two bars in a footing, is common residential practice offered as a starting point. Your building department has the final say, and if the steel is holding the structure up rather than holding a crack together, that is a design from an engineer rather than a number from a calculator.

Limitations

• This page counts steel only. The concrete around it belongs to the concrete calculator, the compacted base under it to the gravel calculator, and the vertical bars and ladder wire in a block wall to the concrete block calculator, which already computes them.
• Spacing, bar size, lap length, edge clearance, chair spacing and the two bar footing default are all common residential practice, offered as editable starting points to confirm with your local building department. They are not a design and they are not engineering advice.
• Suspended slabs, beams, columns, retaining walls, footings carrying point loads and anything on expansive or unstable soil are out of scope. That needs a design from an engineer, not a calculator.
• The cut plan assumes every stick is the same length and that you can cut bar on site or have the yard cut it. If the supplier will only sell full sticks and will not cut, the cut list still tells you what to cut, but the cutting is yours.
• Lap length here is the 40 bar diameter rule of thumb. The real splice length in ACI 318 depends on concrete strength, coating, cover, bar spacing and how many bars are spliced at the same section, and it can be longer than 40 diameters.
• Prices are seeded from big box shelf prices checked on 2026-08-16 for #4 bar, with other sizes scaled from them by weight. Steel prices move. Type your own quotes in, because the ledger is a planning tool and not a quotation.
• Mesh coverage assumes sheets lapped one full square where they meet and no lap at the slab edge. If your inspector wants a wider lap, change the overlap field before reading the sheet count.
• Metric is not offered. This is a US page and every constant in it, from stick lengths to bar designations to the weights, is the US convention.

Frequently asked questions

How much rebar do I need for a 20 by 20 ft slab?

Thirty sticks of 20 ft #4 bar at 16 in on centre, if you take 3 in off each edge. The span is 19 ft 6 in each way, which is 15 bars in each direction and 30 runs, and every run fits inside a 20 ft stick with 6 in to spare, so there are no splices at all. That is 585 linear feet of bar and 390.8 lb of steel. Add 36 chairs and about 113 ties. This is the one common slab size where dividing 585 by 20 also gives 30, so the shortcut and the real answer agree, and the reason is simply that nothing has to be spliced. Add 6 in to the slab and they stop agreeing.

What size rebar do I need for a 4 in slab?

For a residential 4 in slab on grade the common answers are #3 or #4 bar at 12 to 18 in on centre, or welded wire mesh, or fibre in the mix, depending on what the slab carries. #4 at 16 in is the usual default for a garage floor or a driveway, #3 at 16 in is plenty for a patio or a walkway, and mesh alone is normal on a shed pad. What settles it is not the thickness, it is the load and the soil: a slab a car parks on wants bar, a slab a barbecue sits on does not. Confirm with your building department, because plenty of jurisdictions publish a prescriptive detail for exactly this and it beats any rule of thumb.

How far apart should rebar be in a slab?

Twelve to 18 in on centre covers almost all residential slab work, with 16 in the common default. Tighter than 12 in is a sign that a drawing is calling for it, and wider than 18 in is really crack control rather than reinforcement. On a 24 by 24 ft slab the three spacings work out to 64, 48 and 43 sticks of #4, which is $880, $660 and $591 of steel at the seeded price, so the gap between 16 and 18 in is smaller than most people expect, because the bar count drops but the lap and offcut arithmetic barely moves. Every one of those figures is common practice to confirm locally.

How long does a rebar lap splice need to be?

The trade rule of thumb is 40 bar diameters, which gives 15 in on a #3, 20 in on a #4, 25 in on a #5 and 30 in on a #6. The real number in ACI 318 depends on the concrete strength, whether the bar is coated, how much cover it has, how far it is from the next bar and how many bars are spliced at the same point, and it can be longer than 40 diameters. Use 40d as a starting point, put your building department's or your drawing's number in the lap field, and never shorten a lap because the piece you cut was a bit short. A short lap is a hinge, and it is in the worst possible place.

Does a concrete patio need rebar?

Usually not, in the sense that it will still be there in twenty years without it. What a patio needs is crack control, and the cheapest form of that is welded wire mesh plus properly placed control joints. On a 10 by 12 ft patio, mesh is 3 sheets and about $46 of material once you include supports, where a #3 bar grid at 16 in is 13 sticks and about $115. Where rebar earns its money on flat work is anything a vehicle parks on, anything thicker than 4 in, anything over soft or filled ground, and anywhere the slab has to span a soft spot rather than sit on it. Fibre in the mix handles the fine early shrinkage cracking and replaces neither of the other two.

How much does rebar weigh?

Per foot: #3 is 0.376 lb, #4 is 0.668, #5 is 1.043, #6 is 1.502, #7 is 2.044, #8 is 2.670, #9 is 3.400, #10 is 4.303 and #11 is 5.313. So a 20 ft stick of #4 is 13.4 lb and a 20 ft stick of #5 is 20.9 lb. A 48 stick order of #4 is 641 lb, which is inside a half ton pickup's payload on paper and still not something you can carry, because a 20 ft stick overhangs every pickup bed made. The weight only mode on this page turns any list of bars into feet, pounds and tons, and the printable chart below carries diameters, areas, weights and lap lengths for every size from #3 to #11.

Why does this calculator say more sticks than the others?

Because bar cannot be joined end to end. Every calculator on the first page of this search divides your total linear feet by the length of a stick, which answers the question of how many sticks weigh as much as your grid. What you actually have to buy is different for two reasons: every splice overlaps two bars by the lap length, which is real steel that advances the run by nothing, and the leftover tails have to come out of somewhere. On the default 24 by 24 ft slab that is 43 against 48. Both numbers are printed side by side here so you can see which one you have been ordering from.

Do I need rebar in a concrete block wall?

Usually yes, as vertical bars grouted into the block cores and often horizontal ladder or truss wire in the bed joints, but that is a different calculation and it is not this page. The concrete block calculator on this site already computes vertical core bars, ladder wire, block, mortar and grout for a CMU wall, so use that one. This page deliberately does not compute block wall steel, because the two jobs have different spacing rules, different lap rules and a different bill of materials.

Should I buy 20 ft or 10 ft sticks?

Twenty foot sticks are cheaper and need fewer splices, but they will not fit in any pickup bed. On the reference 24 by 24 ft slab, 20 ft stock is 48 sticks, 36 splices and $660.00 of steel; 10 ft stock is 108 sticks, 72 splices and $810.00. So short stock costs about $150 more and puts 60 extra feet of lap steel in the slab, and what it buys you is a load you can drive home. Before you decide, ask the yard whether they will cut 20 ft stock to your cut list, because that is usually cheaper than either option and this page prints the list for you.