How this calculator works
Subtract every post width from the overall run, then divide by the number of equal bays. Within a bay, n balusters make n + 1 clear gaps. The smallest whole count meeting the entered maximum gap is used. A 100 mm starting gap is a planning example, not a statement of a local guard requirement.
Formula
Bay opening = (overall length - posts x post width) / (posts - 1); n = max(0, ceil((opening - maximum gap) / (baluster width + maximum gap))); actual gap = (opening - n x baluster width) / (n + 1).
Worked example
A 2.3 m railing with three 100 mm posts has two 1 m openings. Seven 40 mm balusters in each bay leave eight 90 mm gaps, so buy 14 balusters.
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Frequently asked questions
What if the post bays differ?
Calculate each clear opening separately. Equal-bay division is inaccurate when posts are unevenly placed.
Is spacing measured on centre?
The limit and result here are clear gaps between edges. Centre spacing equals the clear gap plus baluster width.
Can I use this on stairs?
Use the spindle spacing page for horizontal projection and sloped rail gaps. This page models a level straight railing.