Deck Calculator: Boards, Framing, Screws and Material Cost
Estimate the deck boards, joists, beams, posts and screws a deck needs from its size and spacing, and what the materials will cost. Enter the deck size, the board and the framing you plan, add prices from the yard, and take the list to the order or the quote.
Your numbers
Length runs along the deck boards (usually along the house); depth runs along the joists, out from the house.
Optional, for an L-shaped deck or a step-down. Framed on its own and added.
A nominal 6 in board is 5-1/2 in wide.
Typical; follow the board maker's instructions.
The decking sets the limit: see IRC Table R507.7 or the composite board's label.
Off: freestanding, with a beam on both sides.
The span you picked from the joist span table (IRC Table R507.6) for your joist size and species.
The length you buy joists, rims and beams in.
IRC R507.7: two screws or nails per board at each support. For hidden clips, use the clip maker's count.
Typical 5 to 15%, added to boards and fasteners.
Deck boards to buy
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Joist and beam spans, post sizes and footings come from the span tables (IRC R507) and the local code, not from this calculator.
IRC R507.3.2: footings at least 12 in below undisturbed ground, and below the frost line where the deck is attached to the house.
An estimate for the order and the quote. Codes vary by place: the local code and your inspector have the final word.
The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.
How to calculate deck boards
Deck boards run along the deck's length and stack up across its depth. Divide the depth by the board width plus the gap to get the number of rows, round up, and multiply by the boards each row needs: the length divided by the board length, rounded up. Use the actual width: a nominal 6 in board is 5-1/2 in wide.
Worked example: a 16 × 12 ft deck is 192 sq ft. With 5-1/2 in boards and a 1/8 in gap, 144.125 ÷ 5.625 = 25.6, so 26 rows. Each row is one 16 ft board, so 26 boards; with 10% extra for cuts, splits and the odd bad board, order 29. On a 20 ft deck with 16 ft boards each row needs two boards, and the joints should land on a joist.
Joists, beams, posts and footings
Joists run across the depth, one every spacing along the length plus one at the end: a 16 ft deck at 16 in on center has 13. The rim joist runs along the outside edge, and an attached deck hangs its other end on a ledger bolted to the house; a freestanding deck gets a beam on both sides instead. The calculator counts each run in pieces of the lumber length you buy.
Beams carry the joists and posts carry the beams. How far a joist may span between ledger and beam, how far a beam may span between posts and how big the posts and footings must be all come from the span tables in IRC section R507 and the local code, so the calculator asks for the spacing you picked rather than guessing it. Footings sit at least 12 in below undisturbed ground, and below the frost line where the deck is attached to the house. Use the concrete calculator for the footing concrete.
Joist spacing: the decking sets the limit
Under IRC Table R507.7, 5/4 (1-1/4 in) wood decking laid square to the joists across three or more joists may sit on joists up to 16 in on center; laid diagonally, 12 in. 2x decking may span 24 in either way. Composite and plastic boards must carry their allowable span on the label (IRC R507.2.2), so read it before you set the spacing.
Tighter spacing means more joists and a stiffer floor. Going from 16 in to 12 in on the 16 ft example adds four joists and 26 × 4 × 2 = 208 screws, so price both if the customer wants a firmer feel or a diagonal pattern.
Screws or hidden fasteners
The IRC (R507.7) asks for at least two 8d threaded nails or two No. 8 wood screws per board at each support. Every row crosses every joist, so the count is rows × joists × 2: 676 for the example, about 744 with 10% extra. Put the box count in to get the number of boxes.
Hidden clip systems sit in grooved board edges and use their own count per crossing. Enter the maker's number per board per joist, and check that the clip is approved for the board you are quoting.
On the site visit
Measure the deck where it will be built, note which way the boards run and how high it sits above the ground at the far edge. Height decides post length, railing and whether stairs are needed; the stair calculator handles the steps.
Then write down what the numbers miss: board species or composite colour, picture framing or a border, railing style, skirting, lights, and when the customer wants it done. Those details move the quote as much as the board count.
Sources, checked October 2026. All figures are estimates; the local code decides:
More for deck builders and lumber-yard sales: Building materials sales, Distributor sales, Construction daily report.
The list is the easy part of the deck quote
The size comes from the tape measure on the site visit. The board colour, the railing, the stairs, the fascia and when the customer wants it built come from the conversation. Record the walk-through in ParrotNotes and get the summary and next steps, so the quote matches what the customer asked for.
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