How to work out how many decking boards you need
Decking is bought as boards, not square metres, so the useful question is how many boards — and that depends on the gap between them as much as on the size of the deck. This decking calculator counts the rows, the boards in each row, the joists underneath and the screws that hold it all together.
Start with the covered width
A board covers its own width plus the expansion gap beside it. A 144mm board with a 5mm gap covers 149mm of deck, so a 3m wide deck needs 3,000 ÷ 149 = 20.1, rounded up to 21 rows.
Then count boards per row
Each row runs the length of the deck. A 4m deck laid with 3.6m boards needs two boards per row, with the second cut down — which is where most of the offcuts come from.
Add wastage last
21 rows × 2 boards is 42 boards. With 10% for cuts and the occasional warped board, the order becomes 47. Wastage goes on the board count, not the area, because you buy whole boards.
Worked example: a 4m × 3m deck
144mm boards, 3.6m long, 5mm gaps, joists at 400mm centres, 10% wastage.
| Item | How it is worked out | Quantity |
|---|---|---|
| Deck boards | 21 rows × 2 boards + 10% | 47 boards |
| Board length total | 47 × 3.6m | 169.2 m |
| Joists | 4m ÷ 0.4m, plus one end joist | 11 × 3.00 m |
| Joist length total | 11 × 3m | 33.0 m |
| Decking screws | 42 boards × 11 joists × 2 | 924 screws |
12 m² of deck. Narrower 120mm boards would need 24 rows and 53 boards for the same deck — more boards, more screws, more labour.
Joists, spacing and span
Why spacing matters
Joists at 400mm centres are the usual starting point for softwood boards laid straight across. Widening them to 600mm cuts this deck from 11 joists to 7 and the screws from 924 to 588, but the boards flex more between supports, and most manufacturers’ warranties set a maximum span.
Composite and diagonal decks
Composite boards are generally less stiff than timber and need closer spacing. Laying boards diagonally increases the span between joists by about 40%, so the spacing has to tighten to compensate. Check the board’s span table before you buy the frame timber.
The gap between boards
Wet timber shrinks
Pressure-treated boards often arrive damp and shrink as they dry, so a 5mm gap set on wet boards can open to 8mm or more. Dry boards, or boards stored under cover for a few weeks, can be fixed at the gap you actually want.
The gap changes the board count
The gap is part of the covered width, so it changes how many rows you need. Going from 5mm to 10mm on this deck drops the covered width from 149mm to 154mm — fewer rows, slightly fewer boards, and a deck that drains better.
What else the job needs
Frame and base
Beyond boards and joists you need posts or bearers, joist hangers or brackets, and something solid under them. Concrete pads or post holes are priced with the concrete calculator, and a compacted sub-base under a ground-level deck with the gravel calculator.
Fixings and finishing
Use decking screws rated for outdoor use — stainless or coated — because ordinary screws stain treated timber. Add fascia boards for the edges, and a cut-end treatment: every cut exposes untreated timber, which is where rot usually starts.
Before you order
Measure the deck you can actually build, not the one you sketched: check where the door threshold sits, how the deck meets the lawn, and whether a step is needed. Decks over 300mm high, or those that take up a large share of a garden, can need planning permission, and decking attached to a building may fall under Building Regulations.
For the wider budget, including the bits that are easy to forget, see budgeting a renovation. To measure an awkward-shaped deck, split it into rectangles with the square meter calculator first.