How this is worked out
Rows of boards = deck width ÷ (board width + gap), rounded up. Each row needs deck length ÷ board length boards. We multiply those, add the offcut allowance and round up.
Joists run across the boards. The count is deck length ÷ joist centres, plus one for the end. Clips are one per board per joist, which is how most hidden-fixing systems work.
The gap and joist spacing default to UltraDecking’s published guidance for domestic decks. Swap in the figures from your board maker’s instructions.
Worked example
A 4m × 3m deck with 140mm boards, 3mm gaps and 3.6m lengths: 3,000 ÷ 143 = 21 rows. Each row is 4m, so 4 ÷ 3.6 = 1.11 boards. 21 × 1.11 = 23.3, plus 10% = 26 boards.
Joists at 300mm centres over 4m: 4,000 ÷ 300 = 13.3, rounded up to 14, plus one = 15 joists of 3m.
Cutting down waste
Board length is where most money goes. If your deck is 3.5m long, 3.6m boards give almost no waste, while 4.8m boards leave a 1.3m offcut on every row. Try both lengths in the calculator and see which gives fewer boards.
What this doesn’t cover
Rim joists, fascia boards, posts or pedestals, and the frame’s own fixings are extra. Raised decks over about 600mm need a properly designed frame and may need building control.
Questions people ask
How far apart should joists be for composite decking?
UltraDecking says 300mm centres for domestic decks and 200mm for commercial or diagonal boards. Some other brands allow 400mm, so follow the instructions for the boards you buy, because the warranty depends on it.
What gap do composite boards need?
UltraDecking gives 3mm side to side and 6mm end to end, rising to 6 to 8mm at the ends in warm weather and 8 to 10mm below 10°C. Composite expands and contracts more than timber along its length.
How many boards do I need for a 4m × 3m deck?
With 140mm boards, 3mm gaps and 3.6m lengths, 3m across needs 21 rows. Each row is 4m long, so about 1.1 boards a row, which comes to 26 boards with 10% for offcuts.
Do I need planning permission for decking?
In England decking is usually permitted development if it’s no more than 30cm off the ground and, with other extensions and outbuildings, covers no more than half the garden. Raised decks and listed buildings need permission.
Sources
Trade review pending. A named tradesperson will check this calculator, and their name and qualifications will appear here.
Figures last checked 25 October 2026. Read how we calculate and our disclaimer. Found a mistake? Tell us.