Guardrail Baluster Spacing Calculator

Baluster count, actual clear gap, and on-center layout for one rail opening — sized so a 4″ sphere cannot pass (IRC R312.1.3)

Starting point only — not a substitute for the adopted code or the inspector’s sphere. IRC R312.1.3 requires that guards not have openings from the walking surface to the required guard height that allow passage of a 4″-diameter sphere. Limited exceptions exist for the triangular opening at the open side of a stair (6″) and for the open side of stair treads in some editions (4-3/8″). Jurisdictions amend this. Commercial work follows the IBC. Spacing math also says nothing about guard height (R312.1.2) or the 50 lb concentrated infill load in Table R301.5 — a perfectly spaced baluster that pulls out of the rail is still a failed guard.
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