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Baluster Calculator

Use the baluster calculator to estimate how many balusters you will need for your railing.

Codes generally require that a 4 in sphere cannot pass through.
Leave at zero to skip the costing.
Clear
Balusters per section1314 gaps of 3.75 in
Gap between balusters3.75 inwithin the 4 in limit
On-centre spacing5.25 inmark this distance from centre to centre
Gaps14always one more than the balusters
Total balusters131 section
Clear span72 in6 ft
Baluster width taken up19.5 in27.1% of the span
With one fewer baluster4.1538 in gaptoo wide — the count above is the minimum

The method

gaps = balusters + 1; every gap must be under 4 inches

The four-inch sphere rule

Building codes almost universally require that a 4-inch sphere cannot pass through any opening in a guard. It is a child-safety rule, and it applies to the gaps at the ends as well as between balusters — which is why n balusters give n + 1 gaps that all have to comply.

Work out the on-centre spacing and mark from there rather than measuring each gap in turn. Measuring gap to gap accumulates error, and the last one always ends up wrong.

Note that adding one more baluster than the minimum is free insurance: it tightens every gap and costs one stick of wood.