Decking calculator
Work out how many deck boards and joists you need for a deck.
Uses only the sizes you enter.
Show the workings
| Step | Sum | Answer |
|---|---|---|
| Rows of boards across the deck | (3,000 mm + 5 mm) ÷ (144 mm + 5 mm gap), rounded up | 21 rows |
| Boards along each row | 4 m ÷ 3.6 m boards, rounded up | 2 boards |
| Boards | 21 × 2 | 42 boards |
| Joists, one at each end and then at your spacing | 4,000 mm ÷ 400 mm, rounded up, + 1 | 11 joists |
What this leaves out or assumes
- Boards run along the length of the deck. Joins between boards are not allowed for: add a few if your boards will not divide evenly.
Where the figures come from
This sum uses only the numbers you entered.
How many deck boards do I need?
- Divide the deck's width by the width of a board plus its gap to get the number of rows, rounded up.
- Divide the deck's length by the length of a board to get the boards in each row, rounded up.
- Multiply the two.
The gap and the joist spacing depend on the boards: their maker's fitting guide gives the gap to leave and the widest spacing the boards can span.