Calculate decking boards, joists, posts, and hardware for building a deck.
This deck calculator (decking calculator) works out how many deck boards, joists and fixings you need for a deck of a given size, allowing for board width, gaps and waste. Results are in metric and imperial so it suits builders on either side of the Atlantic.
| Item | Typical value |
|---|---|
| Joist spacing (on centre) | 16 in / 400 mm |
| Board width | 5.5 in / 140 mm |
| Board gap | 3–6 mm |
A 4 m × 3 m deck with 140 mm boards laid along the 4 m length needs about 3 m ÷ 0.146 m (board + gap) ≈ 21 rows of 4 m boards, plus a 10% waste allowance for cuts and end joints.
Divide the deck width by the board width plus a small gap (1/8" or 3 mm), then add 10% for waste and cuts.
16 inches (400 mm) on centre is standard. Some composite decking may allow 12 inch (300 mm) spacing.
Posts are typically placed every 6 feet (1.8 m) in both directions, plus at corners and beam ends.
100 × 100 mm (4×4 in) posts suit most low decks; taller or larger decks often need 150 × 150 mm (6×6 in) posts. Spacing is typically 1.8–2.4 m.
The frame, posts and footings often account for 30–50% of materials. Use our Decking Cost calculator to price the whole build.
