How the baluster calculator works
A guard infill is a row of solid spindles and empty gaps. Between two posts there is always one more gap than there are balusters, because the run starts and ends with a gap against a post. If the span is S, the baluster width is b and the count is n, every gap equals (S − n × b) ÷ (n + 1). The calculator looks for the smallest n that brings that gap down to the maximum you entered, then gives you the exact gap so the spacing is equal on both ends.
Balusters per section = ⌈ (span − max gap) ÷ (baluster width + max gap) ⌉The gap is then recalculated from the rounded count. Marking out that recalculated figure, instead of the rule of thumb, is what keeps the last space next to the post from ending up visibly wider or narrower than the others.
Worked example
A landing guard has three sections, each 1.8 m (180 cm) between posts, with 4 cm square spindles and a 10 cm limit. (180 − 10) ÷ (4 + 10) = 12.14, so each section takes 13 spindles. Thirteen spindles fill 52 cm, leaving 128 cm split into 14 gaps of 9.14 cm. The on-centre spacing is 9.14 + 4 = 13.14 cm. Three sections need 39 spindles; with a 5% spare for a split or a knot in the wrong place, order 41. On an imperial deck with an 8 ft span, 1.5 in balusters and a 3.75 in target, (96 − 3.75) ÷ 5.25 = 17.6, so 18 balusters with gaps of 69 ÷ 19 = 3.63 in.
Opening limits you will meet
| Situation | Usual limit | Remarks |
|---|---|---|
| US houses and decks (IRC) | 4 in sphere must not pass | Design to 3.5–3.75 in to allow for wood movement |
| US stair sides (IRC) | 4-3/8 in sphere | The triangle under the rail and tread: 6 in sphere |
| US commercial (IBC) | 4 in sphere | Guard height 42 in instead of 36 in |
| England and Wales, homes | 100 mm sphere | Guarding 900 mm high on stairs and landings |
| Many continental codes | 100–120 mm | Often also bans horizontal rails a child could climb |
These are common figures, not a substitute for the rule where you live. Guard height, the load a railing must resist and how posts are fixed to the structure are set by your local building code; for a raised deck or a landing more than about 1 m above the floor, have the post fixings checked by an inspector or engineer.
Layout tips and mistakes to avoid
- Measure the clear span, not post centres. Using centre-to-centre adds a whole post width and can push the last gap over the limit.
- Use a spacer block. Cut a scrap to the exact gap from the calculator and use it between every pair; a tape measure lets small errors accumulate across 13 spaces.
- Stairs: measure horizontally along the nosings. Most treads take two spindles (three on deep treads) to stay under the stair limit.
- Turned spindles are narrowest at the neck. The gap is measured at the widest opening, so use the thinnest section as the width.
- Metal balusters are often 13–19 mm (1/2–3/4 in), so a section needs noticeably more of them than of chunky timber.
Frequently asked questions
How many balusters do I need for a 6 ft railing?
With 1.5 in balusters and a 3.75 in maximum gap, a 72 in clear span needs 13 balusters, which gives 14 equal gaps of 3.75 in. Use 14 if you prefer a safer gap of about 3.4 in.
What is the 4 inch rule for balusters?
US residential codes say a 4 in sphere must not pass through any opening in a guard, so the clear gap between spindles has to be less than 4 in. Designing to 3.5–3.75 in leaves room for shrinkage.
How many balusters per stair tread?
Two per tread is the usual rule on a standard 10–11 in (25–28 cm) tread; deeper treads or thick balusters may need three to stay under the stair limit of 4-3/8 in in the US.
What is the maximum baluster spacing in the UK?
For homes in England and Wales a 100 mm sphere must not pass through the guarding, so gaps are typically set at 90–99 mm between spindles.
Do I measure baluster spacing centre to centre?
The limit applies to the clear gap between spindles. The centre-to-centre figure is the gap plus one baluster width, which is handy for marking but is not the number the inspector checks.
Last reviewed: 2026-09-27 · How we calculate