Baluster Spacing Calculator
Space balusters or spindles evenly between posts with every gap under the 4 in, 100 mm or 125 mm limit: count, gap, centres and marks.
Guards on stairs and decks must not let a child through, so codes limit the gaps between balusters (spindles): a 4 in sphere must not pass in the US, 100 mm in England and 125 mm in Australia. To space them evenly, find the fewest balusters that keep every gap under the limit: n balusters make n + 1 gaps, so each gap is (rail length − n × baluster width) ÷ (n + 1). On a 72 in rail with 1½ in balusters, 13 balusters leave 3¾ in gaps, with centres every 5¼ in. Enter your rail length, baluster width and the number of sections, and the calculator gives the count, gap, centres and the marks to measure from the post.
- Balusters per section
- 13
- Gap
- 3.75 in
- On centre
- 5.25 in
- Total
- 52
14 gaps
under 4 in
centre to centre
4 sections
Marks from the post face (baluster centres)
4.5 in · 9.75 in · 15 in · 20.25 in · 25.5 in · 30.75 in · 36 in · 41.25 in · 46.5 in · 51.75 in · 57 in · 62.25 in · 67.5 in
How it was worked out
- Fewest balusters with every gap under 4 in: (72 in − n × 1.5 in) ÷ (n + 1) < 4 in → n = 13
- Gap = (72 in − 13 × 1.5 in) ÷ 14 = 3.75 in; centres every 5.25 in
- 4 sections × 13 = 52 balusters
Common use cases
- Space balusters on a deck railing
- Fit spindles to a staircase balustrade
- Check whether an existing railing meets the gap rule
- Order the right number of balusters for every section
How to use
- Choose your units and code; the gap limit fills in (4 in, 100 mm or 125 mm).
- Enter the rail length between the posts and the baluster width.
- Enter how many identical sections you have.
- Read the balusters per section, the gap and the marks from the post face.
FAQ
What is the maximum gap between balusters?
In the US, the IRC's 4 in sphere rule means gaps must be under 4 in. England's Approved Document K uses a 100 mm sphere, and Australia's NCC limits openings to 125 mm. Check your local code.
How do I space balusters evenly?
Work out the count first, then divide the leftover space equally: gap = (rail length − count × width) ÷ (count + 1). Mark the first baluster one gap from the post and step along by the centre spacing.
How many balusters per foot?
For 1½ in balusters with gaps just under 4 in, about 2.3 per foot (one every 5¼ in or so). Wider balusters need fewer.
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Results are estimates for planning and buying materials, worked out from the figures you enter and typical values (weights, coverage, sizes) that vary by product and supplier. They are not engineering, building or professional advice. Check your local building code, the product maker's instructions and your supplier's figures before you buy or build, and use a qualified professional for structural, electrical and gas work. The code checks cover the main dimensions only; the rules where you live, and your building inspector, have the final say.