Baluster Spacing Calculator
Find the number of balusters and the exact gap so your railing is evenly spaced and meets the 4-inch sphere rule.
How to use the Baluster Spacing Calculator
- Enter the distance between posts and pick its unit.
- Enter the baluster width and pick its unit.
- Enter the maximum gap and pick its unit – IRC: openings must not let a 4 in sphere pass.
- Enter the number of identical sections.
- Press Calculate. The result updates automatically as you type.
About the Baluster Spacing Calculator
US residential code requires guards on decks, porches and stairs more than 30 in above grade, with openings small enough that a 4 in sphere can't pass through (4⅜ in on the triangle at stair treads). Because wood and composite parts shift slightly, many builders aim for a 3½–3¾ in gap.
Measure between the posts at the rail, not on center. Start layout from the middle of the section or use a spacer block cut to the actual gap so small errors don't add up at one end. On stairs, measure the gap perpendicular to the balusters, along the rail.
Formula
Balusters n = smallest whole number with (Length − n × Width) ÷ (n + 1) < Max gap
Actual gap = (Length − n × Width) ÷ (n + 1)
On-center spacing = Gap + Baluster width
The gaps are measured between the inside faces of the posts and between balusters; there is a gap at each post. The calculator keeps the gap strictly below the maximum so a 4 in sphere cannot pass.
Example calculation
Using the default values:
- Distance between posts = 72 in
- Baluster width = 1.5 in
- Maximum gap = 4 in
- Number of identical sections = 1
the calculator returns:
- Balusters per section: 13 balusters
- Actual gap: 3 3/4″ (3.75 in)
- On-center spacing: 5 1/4″
- First baluster center from post: 4 1/2″
- Total balusters: 13 balusters
Frequently asked questions
How many balusters do I need per foot?
With 1½ in balusters and gaps just under 4 in, plan on about 2.2 balusters per linear foot of railing.
What is the 4-inch rule?
Building codes require that a 4 in sphere cannot pass through any opening in a guard, to keep small children from slipping through or getting their heads stuck.
How many balusters for a 6 ft railing section?
A 72 in opening with 1½ in balusters needs 13 balusters with gaps of 3¾ in.
Last updated: · Category: Construction calculators