Construction
Deck Board Calculator
Work out how many deck boards and linear feet you need from your deck size, board width and gap.
How the deck board calculator works
This deck board calculator counts the boards you need by working across the deck row by row, then along each row, and finally adds an allowance for waste. It treats the deck as a simple rectangle, so set the length as the direction your boards run and the width as the distance across the boards.
The method has four steps:
- Rows = ⌈ (deck width in × 12) ÷ (board width + gap) ⌉ — how many board-widths, plus their gaps, fit across the deck.
- Linear feet = rows × deck length — total length of decking laid end to end.
- Boards = ⌈ linear feet ÷ board length ⌉ — how many stock boards that covers.
- With waste = ⌈ boards × 1.10 ⌉ — a 10% cushion for off-cuts and mistakes.
Worked example: a 12 × 12 ft deck
For a 12 ft × 12 ft deck with standard 5.5-inch boards and a 1/8-inch (0.125 in) gap, the pitch per row is 5.625 inches. Across the 12-foot (144-inch) width that is 144 ÷ 5.625 = 25.6, rounded up to 26 rows. Each row is 12 ft long, so 26 × 12 = 312 linear feet. Using 12-ft boards, 312 ÷ 12 = 26 boards, and adding 10% waste gives 29 boards to buy.
Common deck board widths
| Board type | Actual width | Typical gap |
|---|---|---|
| 5/4 × 6 pressure-treated | 5.5 in | 0.125 in |
| 2 × 6 lumber | 5.5 in | 0.125 in |
| Composite (most brands) | 5.5 in | 0.1875 in |
| Narrow / 2 × 4 | 3.5 in | 0.125 in |
Gaps let decking drain and expand; composite boards usually need a wider gap than wood, so check the manufacturer's spec. Choosing board lengths that match your deck dimension reduces off-cuts and the waste you have to buy.
Frequently asked questions
How many deck boards do I need for a 12×12 deck?
About 29 boards using 5.5-inch boards with a 1/8-inch gap and 12-ft board lengths — that is 26 rows over 312 linear feet, plus a 10% waste allowance. Change the inputs above to match your boards.
How much waste should I add for decking?
10% is a common allowance and what this calculator uses by default. It covers off-cuts at the ends, the odd warped or split board, and cutting mistakes. Complex shapes with lots of angled cuts may need 15%.
Why does the gap matter?
The gap between boards is added to each board's width when working out how many rows fit, so a wider gap means fewer boards across the same deck. Gaps also let water drain and let the boards expand without buckling.
Does this deck board calculator save my entries?
No. Everything is calculated locally in your browser — your input isn't uploaded or stored.
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