Calculate how many deck boards you need based on deck size, board width and the gap between boards.
Enter the deck length and width, the board width, the gap you want between boards, and the length of the boards you are buying. The tool works out how many boards and the total linear length of decking (timber or composite) you need, with a waste allowance for cuts and offcuts.
| Board | Width |
|---|---|
| US composite/timber | 5.5 in (140 mm) |
| UK timber decking | 144 mm (5.7 in) |
| Gap between boards | 3–6 mm (1/8–1/4 in) |
A deck 12 ft × 10 ft with 5.5 in boards and a 1/4 in gap: each board covers 5.75 in, so a 12 ft width needs 144 ÷ 5.75 ≈ 25 rows. At 10 ft long that is 25 boards, plus 10% waste ≈ 28 boards.
Tip: run boards in the direction that needs the fewest joins, and stagger any end joints over a joist so both board ends are supported.
Divide the deck width by the board width plus gap to get the number of rows, then multiply by the number of board lengths needed to span the deck length. This calculator does it for you and adds waste.
A gap of 3–6 mm (1/8 to 1/4 inch) is recommended for drainage and to allow the timber to expand and contract. Composite boards often need slightly larger gaps — check the manufacturer guidance.
Allow 5–10% extra for cuts, offcuts and mistakes. Use the higher end for diagonal or herringbone patterns.
Boards usually run the length of the deck, at right angles to the joists. Laying them diagonally looks good but uses about 15% more material and needs closer joist spacing.
For 144 mm boards with a 5 mm gap you need roughly 6.7 linear metres of board per square metre of deck — the calculator works this out precisely.
