Woodworking & Construction Calculators

Fine woodworking & cabinetry

Baluster Spacing Calculator (4-Inch Sphere Rule)

The tool increases baluster count until equal end-and-middle gaps are ≤ the maximum (default 4 in). Posts are not counted.

Inputs

Units
in

Inside face of post to inside face of post.

in
in

Results

Balusters
17
Spaces
18
Equal gap
3.917in
Meets max gap?
Yes

Recommended gear

Calculation method

Equal gaps mean n balusters create n+1 spaces. The calculator raises n until the gap is within the sphere rule. If the stock is wider than the opening, the result is invalid.

Exact formula

gap = (railLength − n × balusterWidth) / (n + 1). Smallest n ≥ 1 with gap ≤ maxGap and gap > 0.

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 guard sphere rule. Confirm the adopted code. Metric users often target 100 mm.

Do I count the posts?

No. Enter the clear opening between posts only.

What if the last gap is still large?

Add balusters, slim the stock, or split the run with another post.

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