Woodworking & Construction Calculators

Fine woodworking & cabinetry

Baluster Spacing Calculator (4-Inch Sphere Rule)

The tool finds the smallest baluster count that keeps every gap, at both posts and between balusters, at or under the maximum (default 4 in). Posts are not counted.

Inputs

Units
in

Inside face of post to inside face of post.

in
in

Terms of Service

Results

Please tick the box above to display the Result.

Calculation method

Equal gaps mean n balusters create n + 1 spaces, one at each post. The calculator takes the smallest n whose gap is within the maximum, so every valid result meets it. The result is invalid if a baluster is as wide as the opening or the balusters needed to reach the maximum do not fit.

Exact formula

gap = (opening − n × balusterWidth) / (n + 1). n is the smallest whole number ≥ 1 with gap ≤ maxGap: n = ceil((opening − maxGap) / (balusterWidth + maxGap)). Invalid if that n leaves no gap.

Fences vs stairs

The same math spaces fence pickets. Change max gap if the fence is decorative rather than a guard.

Frequently asked questions

Is 4 in always the law?

It is the common US sphere rule for guards (IRC). Other places set their own limit: UK guidance uses a 100 mm sphere, and European rules are typically 100–120 mm. Enter your local limit as the max gap and confirm the adopted code.

Do I count the posts?

No. Enter the clear opening between posts only.

What if the last gap comes out wider on site?

Small errors pile up at the end. Cut a spacer block to the equal gap and check the marks from both posts. Wider balusters need fewer pieces for the same max gap; slimmer ones widen every gap.

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