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Stairs Calc

Baluster (spindle)

The vertical infill posts of a railing; gaps must reject a 4 in sphere.

Balusters — also called spindles or pickets — are the vertical posts that fill the space between a handrail and the stair, stopping anyone slipping through. The IRC requires the gaps to be small enough that a 4 in (102 mm) sphere cannot pass, the same rule that governs open risers and guardrails. Example: over a 48 in (1219 mm) rail span with ¾ in square balusters, you need enough of them that every clear gap stays under 4 in — the baluster calculator solves the exact even spacing for you. On a raking (sloped) rail the gap is measured along the slope. Even, code-tight spacing is both a safety rule and the look most people want.

Try the calculator Baluster Spacing Calculator

Related terms

Guardrail (guard) The barrier protecting an open edge; minimum 36 in high (IRC residential). Handrail The graspable rail along a stair; code height 34–38 in above the nosing line. Newel post The sturdy anchor post at the end or turn of a railing. Riser (board) The vertical board that closes the gap between two treads on a closed-riser stair. Balustrade The complete railing assembly: handrail, balusters, and newel posts together.

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Baluster Spacing Calculator Enter your railing length and spindle width to get the number of balusters, the exact even gap and center‑to‑center spacing — checked against your code’s 4‑inch sphere rule.
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Written by the Stairs Calc editorial team. Methodology and code references: see our methodology.

Built and maintained by builders, drafters and engineers who plan stairs for a living — every code limit is transcribed from the published standard and cited to its exact section.

Last reviewed 2026-06-20 against IRC 2021/2024

Stairs Calc gives accurate geometry and checks it against published building-code limits, but results are estimates for planning. Codes are adopted and amended locally and change over time. Always confirm dimensions against your local adopted code and a licensed professional before you build.