Baluster Calculator

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IRC code requires ≤4 in gap.
Construction

Calculate how many balusters are needed for a railing based on span, baluster width, and spacing code requirements.

Formula

n = ⌈(span + gap) / (baluster width + gap)⌉
  • IRC building code: max 4-inch gap between balusters.
  • Stair balusters: max gap measured at most angled point.

120 in span, 1.5 in balusters, 4 in gap

Inputs
  • Railing Span: 120 in
  • Baluster Width: 1.5 in
  • Max Allowed Gap: 4 in

n = ⌈(120+4)/(1.5+4)⌉ = ⌈124/5.5⌉ = 23 balusters.

Frequently asked questions

What gap is code compliant?
IRC requires balusters to be spaced no more than 4 inches apart — a 4-inch sphere should not pass through.