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Why baluster spacing has a rule
Balusters are the small vertical bars under a handrail on stairs and balconies. Their job is to stop people, especially small children, from slipping through. Many building codes therefore limit the gap between balusters to 4 inches, because a 4 inch sphere, roughly the size of a small child’s head, must not pass through. Even spacing matters too: gaps that vary look sloppy and the widest one is the one that fails the rule.
This calculator divides a rail into even gaps. Enter the rail length between posts, the baluster width and the maximum gap allowed, and it returns how many balusters you need and the even gap between them. The gap it returns is always at or under your maximum, because the count is rounded up, not down.
How to use the calculator
- Measure the rail length between the faces of the posts, in inches.
- Enter the baluster width. Square wood balusters are often 1.5 inches; metal ones can be thinner.
- Enter the maximum gap. Four inches is the common code value, but check your local rule.
A worked example
A 96 inch rail with 1.5 inch balusters and a 4 inch maximum gap needs 17 balusters, giving an even gap of about 3.92 inches. The math works like this: each baluster plus its gap takes 5.5 inches, and 92 inches divided by 5.5 rounds up to 17 balusters. The leftover is shared evenly, so every gap is the same. If you had used 16 balusters, the gap would have been over 4 inches and the rail would have failed the rule at every gap.
Stairs add a twist: the rail runs at an angle, so measure along the slope, not horizontally. The gap rule still applies to the space between balusters measured at right angles to them, and many inspectors check with a 4 inch ball. When in doubt, aim a little under the maximum rather than exactly at it, because wood moves and measuring errors happen.
Tips for a tidy railing
- Mark baluster centres on the rail before you drill or fix anything. Centres are easier to mark evenly than edges.
- Start from the middle and work outward on a long rail, so any tiny remainder splits evenly at both ends.
- Keep the end gaps the same as the rest. A wide gap next to a post is the gap an inspector notices first.
- On stairs, keep balusters vertical even though the rail slopes. Vertical balusters look right and the rule is written for them.
Common mistakes
The most common mistake is dividing the rail by the maximum gap and forgetting the baluster width. That undercounts balusters and leaves gaps over the limit. Another common mistake is measuring post to post on the outside instead of between the faces, which adds inches that are not really there. A third mistake is rounding down: when the count is not a whole number, always round up, because one more baluster is cheap and a failed inspection is not.
Remember that this tool is an educational guide only. Baluster rules, handrail heights and guard requirements differ by place and by stair versus balcony. Your local code governs the final railing. Use this calculator to plan the count and the spacing, then build to the code.
Newel posts anchor the whole railing, so give them the same care as the spacing. A newel that wobbles makes every baluster beside it feel loose, however even the gaps are. Fix newels solidly to the floor or stair structure, not just to the tread surface, and check them before the balusters go in. Once the handrail is on, test the railing with a firm push: it should feel like part of the house, not like furniture that might move.
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