Deck Board Spacing Calculator

Figure out how many deck (or fence) boards fit across a given width and the exact gap size for an even, symmetric layout.

Available Width (in)

Board Width (in)

Target Gap (in)

How the board count and gap are found

Each board occupies its own width plus one gap, except the last one, which needs no gap after it. That gives a count, and whatever width is left over is then shared out evenly between the joints:

boards = floor((available + gap) / (board width + gap))
actual gap = (available - boards × board width) / (boards - 1)

The defaults are a 120" opening, 5.5" boards and a 1/4" target gap. (120 + 0.25) / 5.75 = 20.91, so 20 boards fit. They cover 110", leaving 10" to split across the 19 joints - an actual gap of 0.526". The target is only used to decide how many boards fit; the gap you get back is what makes them land flush with both edges.

Measuring and choosing a gap

Available width is the finished face of the deck, measured across the joists between the outer edges. Subtract the fascia thickness if the boards run inside it.
Board width is the actual width: 5.5" for nominal 2 × 6 or 5/4 × 6 decking, 3.5" for a 2 × 4.
Dry or kiln-dried boards take a 1/4" gap. Wet pressure-treated stock is laid nearly tight, because a soaked 5.5" board can lose an eighth of an inch as it dries.

Composite and PVC decking has its own published gap, usually 3/16" to 1/4" on the sides and wider on the ends. Follow the maker's figure - getting it wrong voids the warranty.

Frequently asked questions

How big should the gap between deck boards be?

A quarter inch for dry lumber and composite - wide enough to drain and to pass a leaf, narrow enough that a heel will not catch. Wet treated boards go down almost touching and open up to roughly that as they season.

How many deck boards do I need for a 10 ft width?

Twenty 5.5" boards across 120", with about a half-inch gap between them. Then multiply by the number of rows your deck length needs.

Why is my actual gap wider than the gap I asked for?

Because the leftover width has to go somewhere. Once the whole boards are counted, the remainder is divided between the joints so the run finishes flush at both ends. If that gap comes out too wide, rip the two outside boards narrower and lose the difference at the edges instead.