Decking Calculator - Boards and Joists You Need (UK)
Tell us the size of your deck and we'll work out the boards and joist timber you need, with a little spare for cuts.
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How to use this calculator
Enter the length and width of the deck you're building. We'll work out how many 2.4m decking boards you need (including 10% for cuts) and roughly how many metres of joist timber for a frame at 400mm centres.
How we calculate this
- Deck area = length × width.
- A 2.4m board covers about 0.3576 m² (2.4m long × 0.149m wide, allowing a 5mm gap).
- Boards = area ÷ 0.3576, plus 10% wastage, rounded up.
- Joists at 400mm centres: runs = (length ÷ 0.4) + 1, then × width for total metres.
Worked example
A 4m × 3m deck is 12 m². 12 ÷ 0.3576 = 33.6 boards bare; add 10% and round up to 37 boards. Joists at 400mm centres give 11 runs × 3m = 33m of joist timber.
UK notes & gotchas
- Standard UK decking boards are 2.4m long and ~144mm wide; we allow a 5mm gap for drainage and movement.
- Joists at 400mm centres suit most domestic decks; drop to 300mm for heavier use or diagonal boards.
- Use treated (tanalised) timber for joists and check ground clearance and ventilation under the deck.
FAQ
How many decking boards per square metre?
Roughly 2.8 boards per m² for standard 144mm boards with a 5mm gap. We add 10% for cuts.
What spacing for decking joists?
400mm centres is standard for domestic decks; 300mm for heavier loads or diagonal boards.
Do I leave a gap between boards?
Yes - about 5mm lets water drain and the timber move. Our figures already allow for this.