Building & DIY

Spindle Spacing Calculator (Balusters, IRC 4-Inch Rule)

Works out how many balusters you need and the even gap between them for a railing. It keeps the gaps under the 4-inch sphere rule from the building code.

How to use
  1. Enter the railing length, baluster width, and your target gap.
  2. Switch between inches and centimeters if you need to.
Estimates only, allow for waste and confirm against local building codes and supplier specs.
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Common questions

What is the maximum gap between balusters?

Less than 4 inches, so a 4-inch sphere can't pass through. Stairs use a slightly larger 4-3/8 inch limit at the triangle opening.

How do I space balusters evenly?

Divide the rail length by your spacing, round to a whole count, then recompute the exact gap so the ends match.

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