How it’s worked out
The boards run along the deck’s length, in rows across its width, with a small gap between rows:
Rows = width ÷ (board width + gap), rounded up Boards = rows ÷ rows cut from each board (or rows × length ÷ board length) Joists = length ÷ spacing, rounded up, + 1 Screws = rows × joists × 2
When a board is longer than the deck, each one gives as many rows as fit; when it’s shorter, rows are joined end to end over a joist. Then the extra for cuts is added.
Worked example
A deck 4 m long and 3 m wide in 144 mm boards with 5 mm gaps takes 21 rows. In 3.6 m boards, joined over joists, that is 24 boards, 27 with 10% extra. With joists every 400 mm it needs 11 joists and 462 screws. Make the deck 3.6 m long and each board covers a whole row: 24 boards with extra.
How to read the result
- Deck boards: the number to buy, with the extra you chose.
- Joists: how many, and their length across the deck; the frame timber adds a rim joist down each side.
- Screws: two at every joist for every board.
Common mistakes
- Mixing up length and width: the length is along the boards.
- Leaving no gap: boards need a few millimetres for water to drain and for the wood to swell.
- Spacing joists too far apart for the board: composite boards often need 300–400 mm (12–16 in); the board maker says.
How to use it
- Type the deck’s length (along the boards) and width.
- Type the board width, gap and length, and the extra for cuts.
- Type the joist spacing from the board maker’s guide.
Questions
How many deck boards do I need?
Divide the deck’s width by a board’s width plus the gap to get the rows, then work out how many boards make each row. A 4 × 3 m deck in 144 mm boards needs 27 boards of 3.6 m with 10% extra.
How far apart should deck joists be?
Often 400 mm (16 in) centre to centre, closer for thin or composite boards. The board maker’s guide gives the figure for their boards.
How many screws per deck board?
Two at every joist the board crosses. The calculator counts them for the whole deck.
More calculators
See all 39 calculators. Everything runs in your browser; nothing you type is sent anywhere.