Baluster Calculator

The formula

A railing splits a fixed clear span into a row of balusters separated by equal gaps. With N balusters there are always N + 1 gaps (one before the first spindle, one after the last, and one between each pair):

• Balusters needed = ceil((span − maxGap) / (balusterWidth + maxGap)), round up so no gap can exceed the code limit.
• Actual gap = (span − balusters × balusterWidth) / (balusters + 1), the leftover space shared evenly across all N + 1 openings.

Rounding the count up means the real gap is always a little smaller than the maximum you entered, which is exactly what you want for a code-compliant guard.

Worked example

A 72 in clear span with 1.5 in balusters and a 4 in maximum gap:

• Balusters = ceil((72 − 4) / (1.5 + 4)) = ceil(68 / 5.5) = ceil(12.36) = 13.
• Actual gap = (72 − 13 × 1.5) / (13 + 1) = (72 − 19.5) / 14 = 52.5 / 14 ≈ 3.75 in.

So 13 balusters with an even 3.75 in gap fill the span with every opening safely under the 4 in limit.

Pitfalls to avoid

• Measure the clear span, not the post centers. The balusters only fill the open space between posts, so use the inside-face-to-inside-face distance.
• Count the end gaps. N balusters create N + 1 gaps, including one against each post, that is why dividing the span by the baluster pitch alone gives the wrong answer.
• Use the actual baluster width. A nominal “2×2” is really about 1.5 in; using 2 in throws off both the count and the spacing.
• Round counts up, never down. Dropping a baluster pushes a gap over the code maximum and can fail inspection.