Baluster Spacing Calculator
This free baluster calculator turns your railing length into a count of balusters (also called spindles) and evenly-distributed spacing that meets the standard 4 in sphere rule — for deck or stair railings.
How many balusters do I need?
This planner splits your railing into even post-to-post sections, then works out the maximum balusters per section that keep every gap at or under your chosen limit (4 in by default), and recomputes the exact even gap.
How baluster spacing is calculated
- Sections: total railing length ÷ post spacing, rounded up.
- Balusters per section: the largest count that keeps every gap ≤ the max gap.
- Actual gap: recomputed so every gap in the section is identical.
Worked example
An 8 ft railing with 2x2 wood balusters (1.5 in actual) and a 4 in max gap: 2 sections (3 posts), 8 balusters per section, 16 total, with an exact 4 in gap achieved.
Frequently asked questions
How many balusters do I need for a deck railing?
Split the railing into post-to-post sections, then fit as many balusters in each section as keep every gap at or under 4 in. An 8 ft railing of 2x2 wood balusters (1.5 in wide) with posts every 6 ft is 2 sections of 4 ft: 8 balusters each, 16 in total, at an exact 4 in gap. A single 8 ft section between two posts takes 17 balusters at a 3.92 in gap.
What is the 4-inch rule for balusters?
Most US residential codes require gaps between balusters (and under the bottom rail) to be no more than 4 in, small enough that a 4 in sphere cannot pass through. It is a child-safety standard.
How far apart should deck railing posts be?
6 ft center-to-center is a common maximum for residential deck railing posts, though local code and railing material may allow more or require less.
Does baluster spacing differ for stairs?
The same 4 in limit is a common starting point for stairs, and it is this calculator's default. Stair sections are shorter, so the even gap often comes out slightly different. Codes can set different limits on stairs, both between balusters and for the triangle under the bottom rail at each tread, so confirm the numbers with your local building department.
Are metal balusters spaced the same as wood?
Yes. The 4 in maximum gap applies whatever the material; metal balusters are just narrower, so more of them fit in the same railing.
How do I make a baluster spacing template?
Cut a scrap block to the exact gap this calculator shows (for example 3.76 in) and use it as a spacer between balusters as you fasten them. Check the last gap against the post before fastening the final baluster.
Limitations
This tool estimates baluster count and spacing only — not posts, rails or hardware (use the Fence Calculator alongside it). Always confirm your exact local code requirement before building; some jurisdictions differ from the standard 4 in rule.
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Cite this calculator
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