Baluster Calculator

This baluster calculator works out how many balusters or spindles you need to fill a railing, and the even gap to leave between them so the whole run looks tidy and stays within the building code. You enter the clear length of the railing between the posts, the width of each baluster, and the maximum gap you are allowed, then the calculator finds the smallest number of balusters that keeps every gap under that limit and spreads them out evenly. In New Zealand the Building Code clause F4 says a barrier on a deck, balcony or stair must not let a 100 mm sphere pass, so the gaps between balusters have to be less than 100 mm, which is the default here. The maths treats the run as a repeating pattern of baluster then gap, with a gap at each end, so the number of gaps is always one more than the number of balusters. Use it when building or restoring a deck balustrade, a stair rail or a fence, so you can buy the right quantity and set out the spacing before you start fixing timber. Always confirm the gap against your final baluster size and check any local requirements.

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mm
mm
mm
21
balusters needed
Gap between balusters98.2 mm
Number of gaps22
On-centre spacing138.2 mm

Gaps must stay under 100 mm to meet Building Code clause F4. Set out from the centre so the two end gaps match. Estimate only, confirm against your actual timber.

How it works

The run is a repeating pattern of baluster then gap, with a gap at each end, so the number of gaps is one more than the number of balusters. To find the count, the calculator takes the rail length less one maximum gap, divides by the baluster width plus the maximum gap, and rounds up: that is the fewest balusters that still keep every gap under the limit. The even gap is then the rail length minus all the baluster widths, shared across those gaps. On-centre spacing is one baluster width plus one gap, which is the distance from the centre of one baluster to the next.

Worked example

You have a 3000 mm clear railing, 40 mm balusters, and a 100 mm maximum gap. The count is (3000 minus 100) divided by (40 plus 100), which is 2900 divided by 140, or 20.7, rounded up to 21 balusters. With 21 balusters there are 22 gaps, and the timber takes up 21 times 40, which is 840 mm, leaving 2160 mm of gap. Shared across 22 gaps that is 98.2 mm each, safely under 100 mm, and the on-centre spacing is 40 plus 98.2, which is 138.2 mm.

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Frequently asked questions

How many balusters do I need?

Divide the railing length less one gap by the baluster width plus the maximum gap, then round up. For a 3000 mm rail with 40 mm balusters and a 100 mm maximum gap that is (3000 minus 100) divided by 140, rounded up to 21 balusters.

What is the maximum gap between balusters in New Zealand?

The Building Code clause F4 requires that a barrier on a deck or stair does not let a 100 mm sphere pass through, so gaps between balusters must be less than 100 mm. Many builders aim for around 90 to 95 mm to be safely under the limit.

How do I space balusters evenly?

Once you have the count, the even gap is the rail length minus all baluster widths, divided by the number of gaps, which is one more than the baluster count. Setting out from the centre keeps the two end gaps matching.