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Last updated: 3 October 2026
Decking calculator
Work out boards, joists and screws for a new deck. UK units: metres, centimetres and litres. Results update as you type.
How to use this calculator
Measure the deck length in the direction the boards will run, and the width across them. Enter your board width, the gap between boards and the board length.
Joist spacing depends on the board type. Composite boards often need closer spacing, so check the manufacturer's guidance.
How it's calculated
Rows = deck width ÷ (board width + gap), rounded up. Boards = rows × boards per row, plus extra for cuts. Screws assume 2 per board at every joist.
Worked example
A deck 4.8 m × 3.6 m with 144 mm boards and 5 mm gaps needs 25 rows. With 4.8 m boards that's 1 per row, so 25 boards plus 10% for cuts = 28 boards.
Common questions
What gap should I leave between boards?
Follow the board manufacturer. Timber and composite boards need different gaps to allow for drainage and expansion.
Do I need joists at 400 mm or 16 in?
Many boards use that spacing, but some composites need closer joists. Always check the product guidance.
Does this include the frame?
It counts joists across the deck length only. Beams, bearers, posts and fixings for the frame are extra.