Work out evenly spaced balusters or spindles for a railing while keeping gaps within the 100mm (4in) safety limit.
Building regulations require that a 100mm sphere cannot pass through any gap in a guard or balustrade, to stop a small child becoming trapped or falling through. This calculator finds the smallest number of balusters that keeps every gap at or below your limit, then spaces them evenly.
Always measure the actual clear opening between posts, and check the spacing on stairs too, where the rake can open the gaps up. When in doubt, add one more baluster.
In the UK and US the rule is that a 100mm (about 4 inch) sphere must not be able to pass through any gap, so spacing is kept at or below 100mm clear.
Decide the number of balusters, then divide the leftover space equally into the gaps. This calculator does it for you and checks the gap stays within the limit.
No. The gap is the clear space between balusters. We also show the centre-to-centre spacing, which includes one baluster width.
On a raking stair the triangular gaps can be larger than they look, so always check the clear opening to keep within the 100mm safety limit.
Add another baluster. The calculator will flag when the gap exceeds the limit so you can increase the count until it is safe.
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