Baluster Spacing Calculator

How do I space balusters?

in
in
in

Install this many

13balusters

Spaced so the 4 in sphere cannot pass, as IRC R312.1.3 requires for guards, with even gaps at both posts. Stairs and local amendments differ, so confirm with local code.

4 in sphere vs a 3.39 in gap13 balusters at 4.89 in on center
Clear gap3.393 in
On center4.893 in
First center from post4.143 in
Target gap used3.5 in

Get an even layout with no gap that lets a 4 inch sphere through, the rule used for most residential guards.

How it works

Count = (rail length − target gap) ÷ (baluster width + target gap), rounded up. The real gap is what remains shared evenly across all spaces.

Before you order

  • Lay out from the center of a section so the last spaces match at each post.
  • Use a spacer block cut to your exact gap to keep every baluster consistent.
  • Plan on a gap under 4 inches, since wood can shrink and open up after install.

Common questions

What is the 4 inch sphere rule?

Guards must not let a 4 inch sphere pass anywhere between the walking surface and the required guard height. Stair sides and local amendments can differ.

Do I measure from center to center?

The rule applies to the clear gap between balusters. Center-to-center is just a layout number.