What is the maximum gap between guardrail balusters?
Guardrail infill must not allow a 4-inch sphere to pass, so the clear gap between balusters must be less than 4 inches (2021 IRC §R312.1.3, 2021 IBC §1015.4). Because a sphere exactly 4 inches must NOT pass, size the spacing to land just under 4 inches — not exactly at it.
Source: 2021 IRC §R312.1.3 · IBC §1015.4 · Verify against your AHJ and the adopted code edition.
The rule is written as a sphere test, not a fixed dimension: the openings in a guard must be small enough that a 4-inch sphere cannot pass through (IRC §R312.1.3, IBC §1015.4). It’s designed so a small child can’t slip through.
The subtle part is the word “cannot.” A gap of exactly 4 inches would let a 4-inch sphere pass, so it fails. In practice you size the layout for a clear gap strictly less than 4 inches — a hair under, evenly divided across the span so the end gaps at the posts match the gaps between balusters.
Stair exceptions: the triangular opening formed by the tread, riser, and bottom rail on a stair may pass up to a 6-inch sphere, and the openings on the open side of stairs allow up to 4⅜ inches (IRC §R312.1.3).
Work it out: give the guardrail calculator a span and a baluster width and it returns the number of balusters and an even clear gap sized to stay under the 4-inch limit — so you don’t end up one picket short.
Sources
- ICC — International Codes (IRC / IBC) — 2021 IRC §R312.1.3 / 2021 IBC §1015.4