Concrete Calculator - Cubic Yards, Bags & Cost

How to use it

1. Pick the shape you are pouring: a slab, a footing, a round column or tube, a wall, stairs, or curb and gutter.
2. Enter each dimension and choose its unit. Slab and wall thickness are usually in inches, plan sizes in feet; the round-tube diameter is in inches.
3. Set how many identical elements you have, for example twelve deck footings or four columns.
4. Choose a waste factor: 5 percent for tidy ready-mix forms, 10 percent as a standard buffer, 15 percent for an uneven or over-dug subgrade.
5. Pick a bag size, 40, 50, 60 or 80 lb, to see how many bags the same pour would take.
6. Optional: enter a ready-mix price per cubic yard (about $140 is typical), or a price per bag, plus any delivery or short-load fee.
7. Read the headline, cubic yards and bags, then the full ledger below: weight, truckloads and an itemised cost.

At a glance

• 10 x 10 ft slab at 4 in, 10% waste, $140 per cubic yard ready-mix: 33.3 cu ft, about 1.36 cubic yards, or 62 bags of 80 lb mix. Ready-mix works out to about $190 before delivery.
• 24 x 24 ft garage slab at 4 in: 576 sq ft, about 7.1 cubic yards of concrete before waste, roughly 7.8 with a 10 percent buffer. Firmly ready-mix, about one small truck.
• 12 sonotube deck footings, 12 in diameter x 4 ft deep: About 37.7 cu ft, 1.4 cubic yards, or roughly 70 bags of 80 lb mix. At this size a ready-mix short load or a mixing trailer beats hauling bags.

How the math works

Every shape reduces to a volume in cubic feet, then to cubic yards by dividing by 27, because a cubic yard is 3 ft on each side. A slab or footing is length times width times thickness. A round column or sonotube is pi times the radius squared times the height, where the radius is half the diameter. A wall is length times height times thickness. Stairs are treated as a solid pour: each step is a block the full run deep and as many risers tall as its position, so the stepped mass is run times rise times one plus two on up to the number of steps, and the top landing is added as a solid slab the platform depth deep and the full height tall. Curb and gutter is the curb block plus the flat gutter flag in cross section, run along the length. Any element can be multiplied by a quantity. The waste factor multiplies the volume by one plus the percentage before anything is rounded. Cubic feet drive the bag counts: an 80 lb bag yields about 0.6 cu ft, a 60 lb about 0.45, a 50 lb about 0.375 and a 40 lb about 0.30, so bags are the final cubic feet divided by the yield, rounded up. Weight uses 150 lb per cubic foot, which is 4,050 lb or about 2.03 tons per cubic yard. Truckloads divide cubic yards by a full 10 cubic yard truck, rounded up. Cost is either cubic yards times your ready-mix price or bags times your bag price, plus any flat delivery or short-load fee.

1. 1. Get the volume in cubic feet

   Multiply the dimensions of the shape. A slab or footing is length times width times thickness. A 10 by 10 ft slab at 4 in is 10 x 10 x 0.333 = 33.3 cubic feet. A round tube is pi times the radius squared times the height. Keep every dimension in the same base unit; the tool converts inches, metres and centimetres to feet for you.

2. 2. Convert the thickness from inches to feet

   Slabs are measured in feet across but inches deep, and volume needs one unit. Four inches is 4 / 12 = 0.333 ft, six inches is 0.5 ft. Multiplying square feet straight by inches is the single most common concrete mistake, and it overstates the pour by twelve times.

3. 3. Divide by 27 for cubic yards

   Ready-mix is sold by the cubic yard, and a cubic yard is 27 cubic feet (3 ft x 3 ft x 3 ft). So 33.3 cubic feet is 33.3 / 27 = 1.23 cubic yards. This is the raw pour before any waste allowance.

4. 4. Add a waste factor

   Order a little more than the exact volume for spillage, form spread, an uneven subgrade and the concrete you leave in the chute. Ten percent is a sensible default, so 1.23 cubic yards becomes 1.23 x 1.10 = 1.36 cubic yards. Tighten it toward 5 percent for clean forms, raise it to 15 percent over a rough or over-dug base.

5. 5. Convert cubic feet to bag counts

   If you are bagging it, each size makes a known volume of set concrete: an 80 lb bag about 0.6 cu ft, a 60 lb 0.45, a 50 lb 0.375, a 40 lb 0.30. Divide the waste-included cubic feet by the yield and round up. Our slab is 36.7 cubic feet, so 36.7 / 0.6 = 62 bags of 80 lb mix. It takes 45 of the 80 lb bags to make a single cubic yard.

6. 6. Weigh the pour and count truckloads

   Cured concrete runs about 150 lb per cubic foot, which is 4,050 lb (just over 2 tons) per cubic yard. A full ready-mix truck carries roughly 10 cubic yards, so truckloads are the cubic yards divided by ten, rounded up. Anything under a cubic yard is a short load, where the delivery minimum often costs more than the concrete.

7. 7. Price it the way you will buy it

   For ready-mix, multiply cubic yards by the yard price, about $140 in many markets, and add any delivery or short-load fee. For bags, multiply the bag count by the bag price. The tool splits out the waste add-on so you can see exactly what the buffer costs on top of the bare pour.

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.

Bags or ready-mix: watch the one-cubic-yard line

The rough crossover is about a cubic yard, which is 45 bags of 80 lb mix. Below that, a ready-mix short-load fee often costs more than the concrete, so bags win. Above it, the mixing, the bag prices and the cold joints between batches add up, and a truck is cheaper and gives you one continuous, stronger pour.

Order a hair over, never under

Running short in the middle of a pour is the worst case: the first concrete starts setting while you scramble for more, and you get a cold joint or a patched finish. A waste factor is cheap insurance. It is far better to send back a little in the chute than to stop a pour, so round up rather than down.

Pick the slab thickness for the load

A patio or walkway is usually 4 in. A driveway that carries cars is 4 to 6 in, and a slab for heavy vehicles or a garage often 6 in. Every extra inch on a big slab is real concrete: going from 4 to 6 in on a 24 ft square slab adds about 3.5 cubic yards. Match the thickness to the load, not habit.

Account for the subgrade and over-excavation

The tool sizes the form you enter, but a soft or over-dug base swallows extra concrete as the mix slumps into low spots. If the subgrade is uneven, is not compacted, or you dug deeper than planned, raise the waste factor toward 15 percent or measure the real average depth rather than the nominal one.

Mind the specified mix, not just the volume

Volume tells you how much, not which mix. A footing, a driveway and a structural wall may call for different strengths (for example 3,000 versus 4,000 psi) and sometimes fibre or air entrainment. The cubic yards are the same, but order the strength the plan or your local code specifies, not just the cheapest mix.

Stairs are estimated as a solid pour

This tool models steps as solid concrete all the way down, with a solid landing, which matches a typical poured stoop. If your steps are formed hollow, sit on fill, or are cantilevered treads, the real concrete is less, so treat the stair figure as the solid-pour maximum and adjust down for a hollow build.

Common mistakes, and the fix

• × Multiplying square feet by the thickness in inches.

  ✓ Convert the thickness to feet first. Four inches is 0.333 ft, not 4. Skip this and you order twelve times too much concrete.

• × Forgetting to divide cubic feet by 27.

  ✓ Cubic feet are not cubic yards. Divide by 27 to get the yards ready-mix is sold in, or you will wildly over-order.

• × Ordering the exact volume with no waste factor.

  ✓ Keep a buffer, about 10 percent. Spillage, form spread and an uneven base all eat concrete, and running short mid-pour means a cold joint, not just a second trip.

• × Ignoring the short-load fee on a small ready-mix order.

  ✓ Under about a cubic yard, ask the supplier about the short-load surcharge before you book. It can cost more than the concrete, and bags may be cheaper for a small pad or footing.

• × Bagging a job that really needs a truck.

  ✓ Past roughly a cubic yard (45 bags of 80 lb mix), switch to ready-mix. Dozens of bags mean days of mixing and weak cold joints between batches instead of one continuous slab.

• × Mixing feet and inches across the fields.

  ✓ Set the correct unit on every input. The tool lets each dimension carry its own unit and converts them all to feet, so a slab can be feet wide and inches thick without a hand conversion.

Material and unit specifics

Bag yields (40, 50, 60, 80 lb)

A bag of dry mix makes a known volume of set concrete: an 80 lb bag about 0.60 cu ft, a 60 lb about 0.45, a 50 lb about 0.375, a 40 lb about 0.30. So it takes 45 of the 80 lb bags, 60 of the 60 lb, or 90 of the 40 lb to fill one cubic yard. Yields vary a little by brand, so check the bag.

Density and weight

Cured normal-weight concrete is about 150 lb per cubic foot, which is 4,050 lb or roughly 2.03 tons per cubic yard. That is why even a modest slab weighs thousands of pounds and why the subgrade under it has to be firm and well compacted.

Ready-mix truckloads

A full mixer truck carries about 10 cubic yards. Suppliers will bring less, but a load under a truck's minimum (often 1 cubic yard) carries a short-load surcharge. For big pours, order in whole trucks and have enough crew to place each load before it sets.

Mix strength (psi)

Concrete is specified by compressive strength: 2,500 to 3,000 psi for many footings and paths, 3,500 to 4,000 psi for driveways and structural work. Higher strength costs a little more per yard. The volume you need is the same; the strength is a separate choice set by the load and the local code.

Curing basics

Fresh concrete gains strength as it cures, not as it dries, and needs to stay damp. It reaches much of its strength in the first week and roughly full strength by 28 days. Keep it moist and protected from freezing early on, or the finished slab will be weaker than the mix you paid for.

Units: imperial and metric

The defaults are imperial (feet for plan sizes, inches for thickness) because that is how US suppliers sell ready-mix and bags. Each field also accepts metres and centimetres, converted to feet internally, so you can work in whichever system your plans use.

Reading your result

Read the two headline numbers first: cubic yards is what you order from a ready-mix supplier, and the bag count is the same pour done from the store. They already include your waste factor. Use the one-cubic-yard line as your guide: below it, the bag count and any short-load fee usually mean bags are cheaper, and the tool flags the short load; above it, ready-mix is almost always the better buy and the truckloads tell you how many trips it is. The weight is a reminder of how much mass the subgrade has to carry. Cost is only ever the prices you enter, so the estimate is as good as your quote, and the waste add-on line shows what the buffer costs on top of the bare pour.

Limitations

• This is an estimate, not a supplier quote. It does not know your exact mix price, delivery minimum or short-load surcharge.
• It assumes a uniform thickness over the whole pour. For a sloped, dished or over-excavated area, average the depth or split it into sections.
• Bag yields and density are typical figures. Brand, mix design and how wet the concrete is placed move the real numbers slightly.
• Stairs are modelled as a solid pour with a solid landing. Hollow, filled or cantilevered steps use less concrete, so treat the stair figure as the solid maximum.
• It sizes the concrete only. It does not size rebar, mesh, forms, gravel base or the labour, which you plan separately.

Frequently asked questions

How much concrete do I need for a 10 x 10 slab?

A 10 by 10 ft slab is 100 sq ft. At 4 in thick that is 100 x 0.333 = 33.3 cubic feet, or 33.3 / 27 = 1.23 cubic yards. With a 10 percent waste factor, order about 1.36 cubic yards, which is roughly 62 bags of 80 lb mix or a small ready-mix load. At 6 in thick it climbs to about 2.04 cubic yards.

How many 80 lb bags make a cubic yard?

It takes 45 bags of 80 lb concrete mix to make one cubic yard. An 80 lb bag yields about 0.6 cubic feet, and a cubic yard is 27 cubic feet, so 27 / 0.6 = 45. For other sizes it is 60 bags of 60 lb, 72 bags of 50 lb, or 90 bags of 40 lb per cubic yard.

How much does a yard of concrete cost?

Ready-mix commonly runs about $120 to $170 per cubic yard for the concrete itself, with $140 a fair middle estimate, though it varies by region, mix strength and supplier. On top of that expect a delivery charge and, for loads under the truck minimum, a short-load fee that can be $60 to $150 or more. Always confirm the full price with your local supplier.

When should I order ready-mix instead of bags?

The crossover is about one cubic yard, which is 45 bags of 80 lb mix. Below that, bags are usually cheaper because a ready-mix short load carries a surcharge. Above it, mixing dozens of bags means days of work and weak cold joints between batches, so a ready-mix truck is cheaper and gives you one continuous, stronger pour.