Deck Calculator - Boards, Joists, Footings & Cost

How to use it

1. Pick the shape. A plain rectangle needs a length and a width; an L-shape adds a second rectangle for the return.
2. Enter the deck size in feet. Length runs along the house (the ledger side); width runs out from it, which is also the joist span.
3. Choose the decking board: length (8 to 20 ft), face width (5.5 in standard or 3.5 in), and the gap between boards (1/8 in for dry lumber, 1/4 in for wet).
4. Pick the layout. Perpendicular, diagonal or picture-frame each set a sensible waste percent that you can override.
5. Set the joist on-center spacing (12, 16 or 24 in). Closer spacing means more joists and a stiffer deck.
6. Enter the footing size and count: diameter, depth and how many. The tool works out the concrete bags, reusing the same yields as our concrete calculator.
7. Choose hidden clips or face screws, and turn railing on if you need it. Railing auto-measures the perimeter minus the house side, or you can type a length.
8. Pick a material tier to load a starting price, edit any price to your own quote, and read the boards, framing, footings, fasteners, railing and grand total in the estimate slip.

At a glance

• 12 x 16 ft pressure-treated deck, boards perpendicular: About 192 sq ft, 29 boards of 16 ft with 10 percent waste, 13 joists at 16 in on-center, and roughly $1,300 in materials, or about $7 per square foot.
• ~300 sq ft composite deck with a picture-frame border: About 55 to 65 composite boards at 18 percent waste, hidden clips by the bucket, and a material cost several times the pine version. The frame is what drives the higher waste.
• L-shaped deck, a 16 x 12 ft main with an 8 x 6 ft return: About 240 sq ft total, boards and joists summed across both wings, framing estimated as two bays to confirm against your plan.

How the math works

Area is length times width for a rectangle, and the sum of both rectangles for an L-shape. Decking is laid in rows across the width: each board face plus its gap is the row pitch, so the number of rows is the width in inches divided by that pitch, rounded up. Each row is the deck length long, and a row longer than one board is butt-jointed, so boards per row is the length divided by the board length, rounded up. Multiply rows by boards per row for the base count, then add the layout waste (10 percent perpendicular, 15 percent diagonal, 18 percent picture-frame) and round up to whole boards. Joists run out from the house and are spaced along the length, so the joist count is the length in inches divided by the on-center spacing, plus one for the starting joist. The joist span, which the code tables are read against, is the deck width. Rim and end joists are the perimeter band. Blocking rows fall at intervals no greater than 8 ft of span. Footings are cylinders: pi times the radius squared times the depth, times the count, in cubic feet, then converted to concrete bags with the standard dry-mix yields (an 80 lb bag makes 0.6 cu ft). Fasteners are one connection per board per joist, one hidden clip each or two face screws each, rounded up to boxes. Railing wraps the perimeter minus the house side, with posts no more than 6 ft apart and balusters at a 5.5 in pitch for the 4 in code gap. Cost is each group priced on the quantity you need at the price you enter, summed to a grand total and divided by the area for a cost per square foot.

1. 1. Find the deck area in square feet

   Multiply length by width. A 16 ft by 12 ft deck is 192 sq ft. For an L-shape, split it into two rectangles at the inside corner and add them: a 16 by 12 main plus an 8 by 6 return is 192 + 48 = 240 sq ft. Measure to the outside face of the rim, not the house wall, so the boards reach the edge.

2. 2. Count the rows of decking

   Boards sit side by side across the width with a small gap. One standard board is 5.5 in wide, and a 1/8 in gap makes the row pitch 5.625 in. Twelve feet of width is 144 in, so 144 / 5.625 = 25.6, rounded up to 26 rows. A wider gap or a narrower board changes the pitch and the row count.

3. 3. Turn rows into boards, with the run in mind

   Each row is the deck length long. If your board is at least as long as the run, that is one board per row: a 16 ft run with 16 ft boards is 26 boards. If the run is longer than the board, rows are butt-jointed over a joist, so a 20 ft run with 12 ft boards is 2 boards per row. Multiply rows by boards per row for the bare count.

4. 4. Add waste for the layout

   Cut-offs, defects and end trimming mean you buy more than the bare count. A straight perpendicular run wastes about 10 percent; a diagonal run wastes about 15 percent on the angled ends; a picture-frame border wastes about 18 percent because every field board is mitered into the frame. So 26 boards at 10 percent rounds up to 29. Adjust the percent if your cut plan is tighter or looser.

5. 5. Space the joists and read the span

   Joists run out from the house and are spaced along the length. At 16 in on-center, a 16 ft (192 in) deck takes 192 / 16 = 12 gaps, plus one starting joist, so 13 joists. Each joist spans the 12 ft width. That span is the number you check against a DCA6 joist table for your lumber species and size before you commit to the spacing.

6. 6. Add rim, blocking, beams and posts

   The rim and end joists wrap the perimeter: 2 times (length plus width), so 56 lin ft for a 16 by 12. Blocking rows fall at intervals no greater than 8 ft of span, so a 12 ft span gets one interior row, one piece between each pair of joists. Beams carry the joists to the posts, and posts sit along the beam at your chosen spacing, commonly 6 to 8 ft. These are prescriptive counts to confirm with your inspector, not a substitute for sizing.

7. 7. Size the footings in concrete bags

   Each footing is a concrete cylinder: pi times the radius squared times the depth. A 10 in diameter (5 in radius) footing 42 in deep is pi times (5/12 ft)^2 times 3.5 ft = 1.91 cu ft. Three of them is 5.73 cu ft, which is 5.73 / 0.6 = 10 bags of 80 lb mix rounded up. Frost depth sets how deep you dig, so check your local frost line first.

8. 8. Count fasteners, then price every group

   Each board crosses each joist once. Hidden clips use one per crossing; face screws use two. A 26-row, 13-joist deck is 338 crossings, so 338 clips or 676 screws, rounded up to boxes. Then each group, decking, framing, footings, fasteners and railing, is priced on its quantity at the price you enter, summed for the grand total and divided by the area for a cost per square foot.

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.

Get the framing checked, do not trust the count alone

This tool sizes quantities. It does not certify that a 2x8 at 16 in on-center is adequate for your span, species, grade, snow load or hot tub. Joist and beam spans come from the DCA6 tables and your local code, which can be stricter. Pull a permit, have the plan reviewed, and let the inspector confirm the members before you build. The numbers here are a starting point for that conversation, not a green light.

Board face width and the gap you leave

Most decking is a nominal 6 in board with a 5.5 in face; some is a 4 in board with a 3.5 in face, which means more rows and more fasteners for the same deck. Leave a 1/8 in gap for kiln-dried or composite boards that will not shrink much, and up to 1/4 in for wet pressure-treated lumber that will shrink as it dries. Guess the gap wrong and the boards end up too tight or too open.

Joist spacing: 16 in is the default, 12 in for angles

Sixteen inches on-center suits most straight-laid decks. Drop to 12 in when you run boards diagonally, because the board crosses each joist at an angle and spans farther between bearings, or when the manufacturer requires it for a thin composite. Twenty-four inches saves lumber but is only for specific rated boards and short spans. Closer spacing is stiffer underfoot and costs more joists.

Footing depth is set by frost, not by the deck

A footing has to reach below the frost line so it does not heave in winter. That depth is local: a few inches in the deep south, 42 to 48 in across the northern states, and it governs the concrete far more than the deck size does. Look up your frost depth or ask the building department before you set the footing depth, and confirm the diameter your beam and load call for.

Hidden clips or face screws

Hidden clips sit in a groove on the board edge and leave a clean, fastener-free surface, which is why they suit composite and premium boards. They cost more and install slower. Face screws are cheaper, faster and hold better on wood, but leave visible heads. Match the method to the board: most grooved composite is built for clips, while pressure-treated pine is usually face-screwed.

Buy the waste, but not blindly

The waste percent covers cut-offs, factory defects and end trimming. Ten percent is fine for a simple rectangle; raise it for diagonal or picture-frame layouts, lots of angles, or short odd runs where off-cuts do not get reused. Lower it only if you have a tight cut plan and long clean runs. Coming up a few boards short means a second trip and a possible dye-lot mismatch on composite.

Common mistakes, and the fix

• × Treating the framing numbers as engineering.

  ✓ Read joist, beam, post and footing figures as DCA6-style prescriptive estimates to verify. Pull a permit and have the span, species and load checked. A deck failure is a safety issue, not a rounding error.

• × Forgetting the gap between boards when counting rows.

  ✓ Add the gap to the board face for the row pitch. A 5.5 in board at a 1/8 in gap covers 5.625 in per row, not 5.5, so a wide deck needs a row or two fewer than the face width alone suggests.

• × Ordering the bare board count with no waste.

  ✓ Add the layout waste and round up to whole boards. Perpendicular is about 10 percent, diagonal 15, picture-frame 18. Ordering exactly the coverage guarantees a second trip once cut-offs and a couple of bad boards are set aside.

• × Setting footing depth by the deck size instead of the frost line.

  ✓ Dig footings below your local frost depth so they do not heave. In cold climates that is often 42 to 48 in regardless of how big the deck is. Look up the frost line for your area first.

• × Using hidden clips on a board that is not grooved.

  ✓ Hidden clips need a groove in the board edge. Square-edge and most pressure-treated boards are face-screwed instead. Check the board profile before you buy clips, or you will be returning a bucket of them.

• × Butt-jointing decking anywhere but over a joist.

  ✓ Every board end must land on a joist for support. If your board length does not divide the run cleanly, add blocking under the joints or stagger them across rows so no seam is unsupported.

Material and unit specifics

Pressure-treated pine

The budget standard, chemically treated to resist rot and insects. Strong, cheap and widely stocked, but it is wet when bought, shrinks and can cup as it dries, and needs cleaning and sealing every year or two. Face-screwed. Roughly $1.50 to $2.50 per linear foot of board.

Cedar and redwood

Natural softwoods that resist rot without chemical treatment, lighter and more dimensionally stable than pine, with a warm look that ages to grey if left unsealed. Redwood is the premium of the two and the priciest natural option. Both are softer than composite and still need periodic sealing to hold color.

Composite (Trex, TimberTech)

A wood-fiber and plastic blend, capped composite has a hard polymer shell for stain and fade resistance. No sealing, splinter-free, and grooved for hidden clips. Entry lines like Enhance are cheaper; premium capped lines like Transcend cost more but carry the best warranties. Heavier than wood and it gets hot in full sun.

PVC (AZEK)

All-plastic boards with no wood content, so they are the lightest and the most resistant to moisture and rot. Top of the price range, with strong fade and stain warranties. Like composite they are grooved for hidden clips and never need sealing, though they can feel less wood-like underfoot.

Board length and butt joints

Decking comes in even lengths, commonly 8, 10, 12, 16 and 20 ft. Pick a length that matches your run so you butt-joint as little as possible: a 16 ft board suits a 16 ft deck with no seam. Every joint must land on a joist, and on composite you should keep to one dye lot so the color matches.

Fasteners and hangers

Face screws are coated or stainless deck screws, two per board per joist, sold by the pound or the box (a 5 lb box is roughly 350). Hidden clips are sold by the bucket, one per board per joist. Joists hang off the ledger in galvanized or stainless joist hangers, one per joist, with the correct hanger nails or structural screws.

Footing concrete

Poured into a tube form or a formed hole, sized by diameter and depth. Bag yields match the concrete calculator: an 80 lb bag makes 0.6 cu ft, 60 lb makes 0.45, 50 lb 0.375, 40 lb 0.30. For more than about ten bags per hole it is worth pricing ready-mix, but most deck footings are a handful of bags each.

Reading your result

Start with the two headline numbers: the decking boards you buy and the estimated grand total, which is the whole project priced at the numbers you entered. The board count already includes your waste percent, and the spare line shows the extra length that waste buys you. Framing, footing, fastener and railing lines are quantities to hand a lumberyard, but read the joist span line as a code check to confirm, not a finished design. The cost per square foot lets you compare board tiers quickly: swapping pine for composite roughly triples the decking line while framing and footings barely move. Every price is your quote, so the total is only as good as the numbers you put in, and the structural figures are prescriptive estimates to verify with your building department before you build.

Limitations

• This is a materials estimate, not a structural design or an engineered plan. Joist, beam, post and footing figures are prescriptive DCA6-style guidance to confirm with your local building department, and are not a substitute for a permit or inspection.
• It does not size lumber. It counts joists at your spacing but does not tell you whether a given board is strong enough for your span, species, grade or load.
• L-shaped framing is estimated as two separate bays and summed, so it runs a little high where the wings meet and shared beams and joists are not deducted.
• Footing depth must be set by your local frost line, which the tool cannot know. Enter the depth your code requires.
• Stairs, multi-level decks, cantilevers, curved edges, fascia and trim, and elevated framing are not modeled. Add those separately.
• Cost uses only the prices you enter, so taxes, delivery, fasteners for railing, hangers and finishing hardware are not included unless you fold them in.

Frequently asked questions

How many deck boards do I need for a given square footage?

Divide the deck width by the board face plus its gap to get the rows, multiply by the run length, then add waste. As a rule of thumb with standard 5.5 in boards at a 1/8 in gap, you need roughly 2.1 linear feet of board per square foot of deck, so a 200 sq ft deck is about 420 linear feet, which is 26 boards of 16 ft before waste. Add 10 to 18 percent for the layout and round up to whole boards.

What joist spacing should I use for a deck?

Sixteen inches on-center is the common default and works for most straight-laid decking. Drop to 12 in on-center for diagonal layouts, because the board spans farther across the angle, and for thinner composite boards that specify it. Twenty-four inches is only for boards rated for it over short spans. Whatever you pick, confirm the joist span against a DCA6 table for your lumber, and follow your local building code, which can be stricter.

How much does it cost to build a deck?

Materials for a simple pressure-treated deck run roughly $7 to $12 per square foot, so a 200 sq ft deck is about $1,400 to $2,400 in materials. Composite roughly triples the decking cost, pushing materials to $20 to $35 per square foot, and premium PVC more. Labor, if you hire out, often doubles the total again. This tool prices materials at the numbers you enter, so plug in local quotes for a real figure.

Do I need a permit to build a deck?

Usually yes. Most jurisdictions require a building permit for a deck attached to the house or above a certain height, and they inspect the footings, framing and ledger connection. Requirements vary by location, so check with your local building department before you start. Treat the framing and footing numbers here as a starting point to bring to that conversation, not as an approved design.

How many footings does a deck need?

It depends on the beam size and the load, but footings sit under the posts along each beam, typically 6 to 8 ft apart, plus the ledger carries the house side. A small 12 by 16 ft deck often has one beam on three footings; a larger or freestanding deck needs more beams and more footings. Confirm the post spacing and footing diameter your plan and code require, then enter that count here for the concrete.