Baluster Spacing Calculator
How many balusters a rail section takes and the exact gap that falls out, solved so no opening passes a 4 in sphere and every gap comes out equal.
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Results
One section, drawn to scale. The dashed circle is the 4 in sphere the code says must not pass.
Working
- n balusters make n + 1 gaps, so span = n x width + (n + 1) x gap
- smallest n for a 3.8 in gap = ceil((72 - 3.8) / (3.8 + 1.5)) = 13
- actual gap = (72 - 13 x 1.5) / 14 = 3.75 in
- on centre = 1.5 + 3.75 = 5.25 in
- level rail, so spacing along the rail equals the on centre figure
- total = 13 x 4 sections = 52
Quick answer
A guard may have no opening that passes a 4 in sphere, and that includes the gaps at each post. With n balusters there are n + 1 gaps, so the count is the smallest n where the span minus the balusters, divided by n + 1, comes out under 4 in. Aim at 3-3/4 in rather than 4 so build tolerance cannot push a gap over.
n = ceil((span - gap) / (gap + baluster width)); actual gap = (span - n x width) / (n + 1)
Worked example
A 72 in clear span between posts, 1-1/2 in square balusters, four identical sections.
Working
- n balusters make n + 1 gaps, so span = n x width + (n + 1) x gap
- smallest n for a 3.8 in gap = ceil((72 - 3.8) / (3.8 + 1.5)) = 13
- actual gap = (72 - 13 x 1.5) / 14 = 3.75 in
- on centre = 1.5 + 3.75 = 5.25 in
- level rail, so spacing along the rail equals the on centre figure
- total = 13 x 4 sections = 52
Thirteen balusters land the gaps at exactly 3-3/4 in, a quarter inch inside the limit. Twelve would give 4-5/32 in, which fails outright. There is no arrangement between the two, which is why the count has to be solved rather than estimated: one baluster is the difference between passing and failing.
Balusters per span, 1-1/2 in stock
Counts for a 3-3/4 in design gap, which keeps a quarter inch of margin against the 4 in limit.
| Clear span | Balusters | Gap | On centre |
|---|---|---|---|
| 36 in | 7 | 3-3/16 in | 4-11/16 in |
| 48 in | 9 | 3-7/16 in | 4-15/16 in |
| 60 in | 11 | 3-5/8 in | 5-1/8 in |
| 72 in | 13 | 3-3/4 in | 5-1/4 in |
| 84 in | 16 | 3-1/2 in | 5 in |
| 96 in | 18 | 3-5/8 in | 5-1/8 in |
| 120 in | 23 | 3-9/16 in | 5-1/16 in |
Clear span is post face to post face. Measure every section: rails that look identical are routinely half an inch apart.
At a glance
| What it does | Solves baluster count and the exact equal gap for the 4 in sphere rule |
|---|---|
| Inputs needed | Clear span, baluster width, design gap, number of sections |
| Main output | Balusters per section and the resulting gap |
| Formula | n = ceil((span - gap) / (gap + width)) |
| Category | Home & DIY |
| Cost | Free, no sign-up, no download |
How to use the Baluster Spacing Calculator
- Measure the clear span from post face to post face, and measure every section rather than assuming they match.
- Enter the baluster width. Turned balusters are measured at the widest point of the block, not the turning.
- Leave the design gap at 3-3/4 in unless you have a reason to go tighter.
- Mark out at the on centre figure from a story stick, working from one post, rather than measuring gap by gap.
- Check the end gaps at the posts. They are openings in the guard like any other.
Frequently asked questions
How far apart should balusters be?
Close enough that a 4 in sphere cannot pass through any opening, which in practice means gaps of under 4 in everywhere including at the posts. Most builders aim at 3-3/4 in so that normal tolerance does not push a gap over the limit.
Why is it called the 4 inch sphere rule?
Because the code tests openings with a sphere rather than a ruler. It comes from child head entrapment: a gap that a 4 in sphere passes can admit a small child's body, and the head then fails to follow.
How do I work out how many balusters I need?
With n balusters there are n + 1 gaps, since there is a gap at each post as well as between them. Take the span, subtract the total baluster width, divide by n + 1, and increase n until the answer drops below 4 in.
Is the spacing different on stairs?
The gaps between balusters are still held to 4 in, but the triangular opening formed by the tread, riser and bottom rail is allowed a 4-3/8 in sphere. Set the balusters out horizontally and let the raked rail decide where the tops are cut.
Should I measure gaps or centres?
Centres. Measuring each gap in turn accumulates error, and it always lands in the final gap, which is the one against a post where it is most visible. Mark centres from a story stick instead.
Does the gap under the bottom rail count?
Yes. Every opening in the guard is tested, including under the bottom rail and at the posts. A generous gap left under the bottom rail for cleaning is a common failure.
Spacing limits follow the International Residential Code, sections R312.1.3 and R312.1.3.1. Guards are an inspected item and local codes may be stricter, particularly for commercial work, so confirm the requirement before ordering a full set of balusters.