Baluster Spacing Calculator
Equal baluster and spindle spacing that passes the 4-inch rule — with a visual layout. Free.
Enter the run between posts, the baluster width and the maximum gap your building code allows (100 mm / 4 in for the sphere rule). The calculator returns the minimum baluster count, the exact equal spacing and a mark for every baluster position.
FAQ
What is the 4-inch sphere rule?
Building codes (IRC and most national codes) require that a 100 mm / 4 in sphere cannot pass between balusters anywhere along a guard or stair rail. The calculator uses your max gap as this limit and spaces all gaps equally below it.
Why are my gaps smaller than the maximum?
Equal spacing looks best and installs fastest. The calculator adds balusters until every gap — including the two gaps next to the posts — is at or below your maximum, then divides the leftover space equally.
Should spacing be measured to center or edge?
Work with edges, not centers: mark the left edge of each baluster from the left post, as shown in the layout above. Centers drift when the baluster width is included by mistake.