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Baluster Spacing Calculator

Works out the minimum number of balusters for a railing bay and the exact spacing to mark them at, so the gaps are all equal, all under the limit, and the same at both ends.

mm

Clear distance between the newel posts or end supports, for one bay. Run each bay separately.

mm

Measured across the face. 32 or 41 mm for timber spindles, 12–16 mm for metal.

mm

UK: a 100 mm sphere must not pass. US IRC: 4 in (102 mm). Enter what applies.

mm

UK: 900 mm on stairs, 1,100 mm on landings and balconies. US IRC: 34–38 in on stairs, 36 in minimum for guards. For checking only.

bays
Balusters per bay
— balusters

Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.

Centre-to-centre spacing — mm
Clear gap between balusters — mm
Gap at each post — mm
Total balusters — balusters
First centre from the post — mm
Setting out —

The formula

balusters = ceiling((run − max gap) ÷ (width + max gap)) gap = (run − balusters × width) ÷ (balusters + 1) spacing = gap + width

n balusters divide a run into n + 1 equal gaps. The first expression is the smallest n for which every gap fits under the limit; the second spreads the run evenly so the end gaps match the middle ones, which is what makes a railing look set out rather than filled in.

What to watch out for

Why the spacing comes out as an odd number of millimetres:

  • Equal gaps need an uneven spacing. The run is rarely a neat multiple of anything. Dividing it evenly gives a spacing like 123.4 mm; using a round 120 mm instead leaves a wide gap at one end.
  • The gap is clear width, not centres. The sphere test is about what a child’s head fits through — the space between balusters, not the distance between their centres.
  • On a stair, measure square to the pitch, and check the triangle at the bottom of the rail where it meets the newel.

The 100 mm sphere (UK) and 4 in (US) are the usual limits, and handrail heights vary by country and by whether it is a stair or a landing. Both are inputs here because the law is the law where you are, not a constant.

Frequently Asked Questions

How far apart should balusters be?

Close enough that a 100 mm (UK) or 4 in (US) sphere cannot pass between them. For 32 mm spindles on a 2,400 mm run that works out at 18 balusters at 128 mm centres, leaving 96 mm clear.

How many balusters do I need?

The smallest number that keeps every gap under the limit: (run − max gap) ÷ (baluster width + max gap), rounded up. A 2,400 mm run with 32 mm balusters and a 100 mm limit needs 18.

How high should a handrail be?

UK: 900 mm on a stair, 1,100 mm on landings and balconies. US IRC: 34–38 in on a stair and a 36 in minimum guard elsewhere. Commercial and public buildings often require more. Enter the figure that applies to you.

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