Decking board calculator

Board count on a deck isn't just area divided by coverage — it depends on how the boards are laid out in rows, and rows rarely divide evenly into a deck's width. Order by area alone and you're usually short a board or two once the last, narrower row is cut in. This calculator works from the actual row layout instead.

Why the gap between boards matters

Decking boards aren't laid edge to edge — a small gap, typically around 5mm, is left between each one for drainage and to allow the timber to expand and contract with the weather. That gap adds up: on a wide deck it can account for a full extra row's worth of space over the length of the job, so it has to be counted, not ignored.

Board width changes the row count

Wider boards cover the same width in fewer rows, which means fewer joints and a faster lay — but narrower boards can look better on smaller decks and are sometimes cheaper per board. This calculator assumes boards run the full length of the deck; if your layout runs boards the other way, just swap which dimension you enter as length and which as width.

A worked example

A 4m × 3m deck in 145mm boards needs 20 rows, 80 linear metres of board, and 23 boards at 3.6m each. Drop to 120mm boards on the same deck and the row count climbs to 24, needing 27 boards — narrower boards mean more joints and more waste on the same area.

What this doesn't cover

Joist spacing and the substructure underneath aren't part of this figure, and neither are fixings, end-caps or trims. Staggering joints on longer runs — good practice for strength and appearance — uses more offcuts than a single straight cut, so build in a bit of extra margin if that's how you're laying it.