How it works
The formula
gap = (span − balusters × width) ÷ (balusters + 1)
balusters = the fewest that make every gap 3 3/4 in or less
balusters = the fewest that make every gap 3 3/4 in or less
- We plan gaps of at most 3 3/4 in, so small cutting errors or a board that moves still pass. The International Residential Code (R312.1.3, 2021 edition as adopted) says a 4 in sphere must not pass through a guard, so every gap is under 4 in. Along the open side of stairs it allows 4 3/8 in, and the triangle under the bottom rail at a stair can be up to 6 in.
- Guards are required where a deck is more than 30 in above the ground, at least 36 in high (34 in along stairs).
- A 2×2 is 1 1/2 in square, by the softwood lumber standard (PS 20); check the width of the balusters you buy.
Balusters per section by span
| Span | 1 1/2 in (2×2) | 3/4 in (metal) |
|---|---|---|
| 4 ft | 9 (3 7/16 in gaps) | 10 (3 11/16 in gaps) |
| 5 ft | 11 (3 5/8 in gaps) | 13 (3 9/16 in gaps) |
| 6 ft | 13 (3 3/4 in gaps) | 16 (3 1/2 in gaps) |
| 7 ft | 16 (3 1/2 in gaps) | 18 (3 11/16 in gaps) |
| 8 ft | 18 (3 5/8 in gaps) | 21 (3 5/8 in gaps) |
| 10 ft | 23 (3 9/16 in gaps) | 26 (3 3/4 in gaps) |
Questions people ask
How many balusters per foot?
For 2×2 wood balusters, about 2.2 a foot: 13 in a 6 ft section. Thinner metal balusters need a few more, because each covers less of the gap.
What is the 4 inch rule?
Building codes don't let a 4 in ball pass through a railing anywhere, so a small child can't slip through. Plan gaps of 3 1/2 to 3 3/4 in so the finished railing still passes once boards shrink or shift.
How do I space balusters evenly?
Use the on-center spacing: mark the first baluster's center one gap plus half a baluster from the post, then step the on-center distance along the rail. A spacer block cut to the gap makes it quick.