Baluster Calculator
Calculate how many balusters or spindles a railing run needs and the exact even spacing that keeps every gap at or under your limit.
Spacing balusters evenly
A railing looks right when every gap is identical, so the count is not chosen — it is derived. The run is filled with the smallest number of balusters that keeps every clear gap at or under your limit: the balusters take their share of the run, and what is left divides into one more gap than there are balusters (a gap at each end, against the posts).
Balusters = (run − max gap) ÷ (max gap + baluster width), rounded up · Actual gap = (run − balusters × width) ÷ (balusters + 1)
Example: an 8 ft run (96 in) with 1.5 in balusters and a 3.5 in gap limit needs 19 balusters: they take 28.5 in, and the remaining 67.5 in divides into 20 gaps of 3.38 in — a spacing of 4.88 in on centers.
The 4-inch reference
The figure behind the limit is the 4-inch sphere rule, a common reference in US residential codes: guards are built so that a sphere 4 in in diameter cannot pass through any opening. Keeping the clear gap under 4 in — this calculator's 3.5 in default leaves margin for a slightly crooked baluster — is how that is met on a level run. Stair railings are commonly referenced against a larger sphere figure, and codes differ by jurisdiction and change over time; treat these numbers as layout planning and confirm your local requirements for permitted work.
| Run | Baluster width | Max gap | Balusters | Actual gap |
|---|
| 6 ft | 1.5 in | 3.5 in | 14 | 3.40 in |
| 8 ft | 1.5 in | 3.5 in | 19 | 3.38 in |
| 10 ft | 1.25 in | 3.5 in | 25 | 3.41 in |
| 12 ft | 2 in | 3.5 in | 26 | 3.41 in |
Measure the run between the inside faces of the posts, and lay the spacing out on the rail itself with a story pole or a spacer block cut to the on-center figure — measuring each gap fresh accumulates error, and the last gap shows it. Thin metal or cable systems follow different rules entirely; this layout is for vertical balusters and spindles.
Frequently asked questions
Is the gap measured between balusters or center to center?
Both figures are in the result, and they answer different questions. The clear gap — face to face — is what spacing limits refer to. The on-center figure (gap plus one baluster width) is what you mark on the rail when laying out.
Why not just use more balusters for a smaller gap?
You can: lower the widest-gap figure in the form and the count rises with the gaps shrinking evenly. Many people run 3 in gaps for a denser, more traditional look — it costs a few more balusters per run.
Does this work for a stair railing?
The even-spacing maths is the same, but stair runs are measured along the rake (the sloped length), and the opening limits commonly referenced for stairs differ from level guards. Use the sloped length as the run, keep the gap limit conservative, and confirm local requirements.
What about the gap at the bottom rail?
The limit applies to every opening in the guard, including between the bottom rail and the tread or deck. Keep the bottom gap within the same figure as the baluster gaps.
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