Deck baluster spacing calculator: how many balusters

Even gaps between balusters look right and pass inspection. The rule behind it is the 4 inch sphere.

What is the maximum baluster spacing?

The IRC says a railing's openings must not let a 4 in sphere pass. In practice the gap between balusters is 4 in or less. Local codes can differ.

How do you calculate the number of balusters?

For a rail section of length L between posts, baluster width b and a maximum gap g (4 in): the number of balusters n is at least (L − g) ÷ (b + g), rounded up. The actual gap is then (L − n × b) ÷ (n + 1).

Example. A 72 in section with 1 1/2 in balusters: (72 − 4) ÷ (1.5 + 4) = 12.4, so 13 balusters. The gap is (72 − 13 × 1.5) ÷ 14 = 3 3/4 in, and the spacing on centre is 5 1/4 in.

Marking the positions

Work from one post: the first baluster's edge is one gap in, then one baluster width and one gap for each one after. Write the on-centre marks on a story stick rather than measuring each one.

Balusters in Clad

Clad's layout tool gives the number of balusters and the equal gaps for a length, and the on-centre marks, including layouts along a slope for stair rails.

See what Clad does. Clad is coming soon to the iPhone.

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How this page was made

Written with the help of an AI assistant (Claude, by Anthropic). Every worked example was computed again by a program and checked against its formula. Building codes differ by place: check yours before you build.