Baluster Spacing Calculator: 4-Inch Sphere Rule

How to use the baluster spacing calculator

1. Choose a guard (4 inches) or a stair guard (4-3/8 inches, IRC R312.1.3 Exception 2).
2. Enter the clear length between posts and the width of each baluster.
3. The page counts balusters so every gap, including the two end gaps, is no wider than the maximum, and reports the resulting gap.

Fence picket gaps are calculated on the fence calculator. Stair rise and run are on the stair stringer calculator.

The sphere rule and the count formula

IRC R312.1.3 requires that required guards not allow a 4-inch-diameter sphere to pass. IRC R312.1.3 Exception 2 allows a 4-3/8 inch sphere at the open sides of stairs. The count is n = ceil((L − max gap) / (width + max gap)), then gap = (L − n × width) / (n + 1), which includes the end gaps next to the posts.

A worked example: 96-inch opening, 1½-inch balusters

In a 96-inch clear opening with 1½-inch balusters and a 4-inch maximum, n must be at least (96 − 4) / (1.5 + 4) = 16.73, so 17 balusters. The clear gap is (96 − 17 × 1.5) / 18 = 3.92 inches, just inside the 4-inch limit.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Guard mode uses a 4-inch maximum clear gap from IRC R312.1.3 (a 4-inch sphere shall not pass). Stair-guard mode uses the 4-3/8 inch exception in IRC R312.1.3 Exception 2.
• Baluster count n = ceil((L − max gap) / (width + max gap)); gap = (L − n × width) / (n + 1). End gaps next to both posts are included.
• This is not a fence picket-gap calculator. Picket spacing stays on the fence calculator.

Sources

1. City of Seattle, SDCI, 2021 Seattle Residential Code, Chapter 3 — Building Planning (R312). Public reprint of the 2021 IRC guard and stair-guard opening limits: R312.1.3 (4-inch sphere at required guards) and R312.1.3 Exception 2 (4-3/8 inch sphere at the open sides of stairs).

Last reviewed November 12, 2026. The adopted IRC sections and the inspector’s interpretation win.