Deck size
Board and gap
Joists
| Layout | Amount |
|---|
Joists and fixings
| Joists needed | 0 |
| Joist length (each) | 0 m |
| Screws (2 per board per joist, plus 5% spare) | 0 |
How this calculator works
Boards run along the length you enter, laid in rows that stack up across the width. Each row's width is the board width plus the gap between boards, so the number of rows is the deck width divided by that figure, rounded up. Within each row, boards run end to end along the length; if a single board doesn't span the full length, more than one board per row is needed, which this calculator assumes end to end with a butt joint over a joist, the standard approach when a single length won't reach. Joists run the other way, spaced at your chosen centres along the length, with one extra joist to close off each end, and each joist spans the full width of the deck. Screw count assumes the standard two fixings per board at every joist it crosses.
Making the most of full-length boards
Where possible, plan your deck length around stock board lengths (commonly 2.4, 3.0, 3.6 or 4.8 m) so each row uses one board with minimal offcut, rather than two boards butted together. This calculator counts boards assuming butted rows where needed; if you can avoid the joins by choosing a deck length close to a stock length, you'll use fewer boards than the raw count from awkward offcuts, though the total board length purchased is similar either way.
Choosing a wastage allowance
5% covers ordinary cutting waste and the odd damaged board. Increase it for a deck with lots of corners, steps, or a diagonal board pattern, all of which produce more offcuts.
What this calculator does not check
This tool covers boards, joists and fixing screws only. It doesn't check the support structure underneath (posts, post footings, or joist hangers and ledger fixing to a house wall), whether your deck's height or area needs planning permission under permitted development rules (raised decks and larger areas often do), the timber treatment class needed for ground contact, or handrail and baluster requirements once a deck is raised above roughly 300 to 600mm, which building regulations address separately.
Related tools
Setting posts in concrete for the deck's support structure? The concrete calculator's post hole mode works out the mix needed per hole. Building a fence alongside the deck? The fence calculator uses the same post-and-concrete or postcrete approach.