Deck Calculator — Boards, Joists & Posts
A 16 × 12 ft deck takes 29 deck boards, 13 joists at 16" on centre, 3 posts and about 687 screws. Board counts use the actual 5.5" dressed width plus the gap, not the nominal 6" — that difference alone is 6% of the order. Span tables follow AWC DCA 6, verified 2026-08-04.
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Deck Calculator — materials list
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| Item | Quantity | Cost |
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Prepared with MaterialMath — https://estimatormath.com. Quantities are estimates based on the dimensions entered and the published figures cited on the tool page (verified 2026-08-04). Verify against your own measurements and your supplier's actual pack sizes before ordering. Not professional engineering, construction or financial advice.
How is deck material calculated?
A deck is two calculations stacked: the surface, which is an area problem governed by board coverage, and the frame underneath, which is a span problem governed by code tables.
The first line is the one that matters most in practice. Dressed lumber is planed down from its rough sawn size, so a "5/4 × 6" deck board measures 1 inch by 5.5 inches. Add the 1/8" gap you leave for drainage and expansion and each board covers 5.625 inches of deck. Work from the nominal 6 inches and you order 6% short.
Composite is worse, because most brands specify a 3/16" gap rather than 1/8" — the material moves more with temperature. That changes the pitch to 5.6875" and changes the row count on a wide deck.
A worked example: a 16 × 12 ft deck
5/4 × 6 pressure-treated decking in 16 ft lengths, joists at 16" on centre, 10% waste. Boards run the 16 ft direction so each row is a single board with no butt joints.
Step by step
- Deck area
- 16 × 12 = 192 ft²
- Board pitch
- 5.5″ + 0.125″ = 5.625″ = 0.4688 ft
- Rows across the 12 ft width
- 12 ÷ 0.4688 = 25.6 → 26 rows
- Boards with 10% waste
- 26 × 1.10 = 29 boards
- Joists at 16″ o.c.
- 16 ft ÷ 16″ + 1 = 13 joists
- Beam lines
- 1 (12 ft joist span is within the table)
- Posts
- 3 at 8 ft spacing
- Screws
- about 687
- Footing concrete
- 14 × 80 lb bags
Note the row count: 25.6 rounds to 26, which means the last board gets ripped narrower to finish flush. That is normal and it is why the waste allowance exists. If you would rather not rip, adjust the gap slightly — spreading an extra sixteenth of an inch across 26 gaps buys you an inch and a half of deck width.
Who this calculator is for
- DIY deck builders who need a full material list — surface, frame, fasteners and footings — from two dimensions.
- Deck contractors pricing a rectangular deck quickly, with the actual-dimension arithmetic already handled.
- Anyone deciding between wood and composite — the profile dropdown changes the gap and flags the joist spacing requirement, which is often the deciding factor on an existing frame.
Footings feed straight into the concrete calculator, and if you are buying framing lumber by the board foot the board foot calculator will tally the order. Deck stairs are their own problem — the stair calculator checks them against the code limits.
What this calculator does not account for
- Ledger flashing and lateral load connectors. Both are IRC requirements, both are commonly missed, and ledger failure is the leading cause of deck collapse. Not a quantity — a detail, and one worth getting inspected.
- Railings, guards and stairs. Guards are required above 30" above grade, with no opening passing a 4" sphere. None of that is counted here.
- Engineered design. AWC DCA 6 is prescriptive and has limits. A deck over 8 ft above grade, one carrying a hot tub, or one with an unusual shape falls outside it and needs an engineer.
- Frost depth. The footing depth default is a placeholder. Your building department publishes the real figure and it can more than double the concrete.
- Non-rectangular decks. Angled corners, wrap-arounds and picture-frame borders all add waste and cuts. Break the deck into rectangles and run each one.
- Diagonal decking. Laying boards at 45° needs 12" joist spacing and roughly 15% more decking. Change both inputs if you are doing it.
Board coverage by profile
| Profile | Actual width | Typical gap | Coverage per board |
|---|---|---|---|
| 5/4 × 6 (actual 5.5") | 5.50″ | 0.125″ | 5.625″ |
| 2 × 6 (actual 5.5") | 5.50″ | 0.125″ | 5.625″ |
| 2 × 4 (actual 3.5") | 3.50″ | 0.125″ | 3.625″ |
| Composite (actual 5.5") | 5.50″ | 0.1875″ | 5.6875″ |
Dressed dimensions per US Voluntary Product Standard PS 20. Composite uses a wider gap because it expands more with temperature than wood does — check your specific product, since brands differ.
Common questions
How many deck boards do I need for a 12x16 deck?
For a 12 × 16 ft deck in 5/4×6 decking at 10% waste, budget 29 boards. Set your own dimensions and board profile above for an exact count — the actual dressed board width, not the nominal size, drives the real number.
How many screws per deck board?
Two screws at every joist it crosses. A board spanning joists at 16" on centre over a 12 ft width crosses roughly 9 joists, so that one board takes about 18 screws. Multiply by your total board count for the full order — the calculator above does this automatically and rounds to whole boxes.
How many deck boards do I need?
Divide the deck width by the board coverage — the actual board width plus the gap between boards — to get the number of rows, then multiply by how many boards make up each row.
The trap is the word "actual". A 5/4 × 6 deck board is 5.5 inches wide, not 6. With a 1/8" gap it covers 5.625" of deck. Estimating on the nominal 6 inches under-orders by about 6%, which on a 16 × 12 ft deck is two boards short on the last day.
What joist spacing do I need for composite decking?
16 inches on centre for boards laid perpendicular, 12 inches if laid diagonally. Most composite manufacturers require this, and installing at 24" both voids the warranty and produces a floor that flexes underfoot between joists.
Composite is stiffer than wood in bending but far less stiff in shear, and it also creeps under sustained load — a chair or a planter left in one spot will leave a permanent dip at 24" spacing. Wood 2× decking is the only thing that reliably spans 24".
How many screws do I need for a deck?
Two per board at every joist crossing. For the 16 × 12 ft example on this page that is about 687 screws including a 10% allowance — around 350 screws per 100 ft² of deck at 16" joist spacing.
Buy the 5 lb box. Deck screws must be rated for contact with the preservative in modern pressure-treated lumber, which is corrosive to plain steel and to ordinary galvanising. Look for a coating rated for ACQ or MCA treatment, or use stainless.
How deep do deck footings need to be?
Below the local frost line, and there is no national figure. It ranges from no requirement in southern Florida to 8 feet in northern Minnesota. The default in this tool is a placeholder, not a code figure — replace it with the depth your building department publishes.
Footings that stop above the frost line will lift the deck every winter and drop it every spring. Once a deck has heaved unevenly, the ledger connection at the house takes the strain, and that is the connection you least want to load.
How far apart can deck posts be?
It depends on the beam, and the beam depends on the joist span. Under the AWC DCA 6 prescriptive tables, a doubled 2×10 beam carrying a 12 ft joist span can spread posts about 8 ft. A doubled 2×8 carrying the same joists is closer to 6 ft.
This calculator uses the 8 ft figure as a working default. For a real deck, look your species, beam size and joist span up in the DCA 6 tables — they are free, and they are what your inspector will check against.
What does this calculator not cover that will fail an inspection?
Three things, and all three fail regularly. Ledger flashing — the connection to the house must be flashed so water cannot get behind it; rotted ledgers are the single most common cause of deck collapse. Lateral load connectors — required by the IRC to tie the deck back to the house framing. Guards and handrails — required above 30 inches, with a 4" sphere rule on the openings.
None of them are quantities this tool produces, and all of them are code requirements. Get the permit and get the inspection.
Sources for the figures used
Every rate, coverage figure and code limit in this calculator comes from a published source. Where a figure is a convention rather than a standard, it says so.
- AWC DCA 6 — Prescriptive Residential Wood Deck Construction Guide — Joist and beam spans, post spacing, connection requirements (checked 2026-08-04)
- International Residential Code 2021, §R507 — Decks — Ledger attachment, lateral load connection, footing requirements (checked 2026-08-04)
- US Voluntary Product Standard PS 20 — American Softwood Lumber Standard — Dressed dimensions: a 5/4 × 6 deck board is 5.5" wide (checked 2026-08-04)
- Composite decking manufacturer installation instructions — Joist spacing and gap requirements vary by brand — always check the specific product (checked 2026-08-04)