Find out exactly how many deck boards, joists, and screws you need — plus an optional cost estimate.
Joists
10
Screws
580
Calculator
Deck boards
29
Joists
10
Screws (est.)
580
Material breakdown
Boards
29
Joists
10
Screws
580
How to calculate deck materials
Start with the deck boards: divide the deck width by the board width plus the gap between boards (both in the same unit). Round up to get the number of rows. Then divide the deck length by the board length and round up to get boards per row. Multiply rows by boards per row and add your waste allowance — typically 10 % for straight layouts and 15 % for diagonal or herringbone patterns.
For joists, divide the deck span (the direction joists run) by the spacing — 16 inches on-centre is the most common for residential decks — then add one for the starter joist. Use 2 screws per board per joist crossing as a reliable fastener estimate.
What board width should I use?
5/4x6 decking is the most popular residential choice, with an actual face width of 5.5 inches. 2x6 lumber (5.5 in actual) and 2x4 (3.5 in actual) are also common. Always use the actual (dressed) width, not the nominal size.
What gap should I leave between boards?
A gap of 1/4 inch (0.25 in) is the standard recommendation for most climates — it allows drainage and air circulation while preventing tripping. In very wet climates some builders use 3/8 inch.
Why add a waste allowance?
Off-cuts at the ends of runs, boards rejected for defects, and diagonal or angled layouts all increase waste. A 10 % allowance is typical for square decks; bump it to 15 % for diagonal patterns.
Results are estimates. Verify with a professional for important decisions.
About this calculator
This calculator estimates the exact number of deck boards, joists, and screws needed for a wood deck build. Use it before purchasing lumber to avoid under-buying — or over-buying — materials for any rectangular deck.
How to read your results
The headline figure is the total board count, already rounded up and adjusted for your chosen waste percentage. The horizontal bar chart below it compares boards, joists, and screws on the same scale so you can see the relative volume of each material at a glance. If you enter optional per-piece prices, the result card also shows an estimated total material cost.
Worked example
A 16 ft long by 12 ft wide deck, using 5.5 in wide boards that are 16 ft long, with a 0.25 in gap between boards, joists at 16 in on-centre, and a 10% waste allowance.
Real cuts always produce offcuts: boards split along knots, end cuts shorten lengths, and diagonal or picture-frame patterns increase scrap. A 10–15% waste allowance is the industry-standard buffer recommended by the Inch Calculator deck-board method.
What joist spacing should I choose?
The American Wood Council's DCA 6-15 prescribes 12", 16", or 24" on-centre spacing depending on species and span. 16" OC is the most common choice for standard lumber grades; use 12" OC for longer spans or heavier loads.
How is the screw count calculated?
The calculator uses 2 screws per board per joist crossing — one through each edge of the board — which matches the fastener pattern in AWC DCA 6-15 §5. The total is boards × joists × 2.
Does the calculator handle non-rectangular decks?
No. The formulas assume a simple rectangular deck. For L-shaped or irregular decks, split the area into rectangles, run the calculator for each section separately, then add the totals.
Should I use the board width with or without the gap?
Enter the actual board face width (e.g., 5.5 in for a nominal 2×6) and enter the desired gap separately. The calculator combines them internally to compute the centre-to-centre spacing of each row.
How it's calculated
Board count is derived from the Inch Calculator deck-board method. The number of rows is ceil(deckWidth / ((boardWidth + gap) / 12)), converting inches to feet before dividing. Boards per row equals ceil(deckLength / boardLength). Both are multiplied together, then multiplied by (1 + wastePct / 100), and the result is rounded up to a whole board. Joist count follows AWC DCA 6-15 prescriptive framing: count = floor(deckLength / (spacingIn / 12)) + 1, where the +1 adds the starter joist at the ledger or beam end. Screw count is boards × joists × 2, reflecting two fasteners per board-joist crossing as specified in AWC DCA 6-15 §5.
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