Baluster Spacing: Code Rules and Equal Layout
The 4-inch sphere rule, how many balusters you really need, and how to mark equal spacing without drift.
Start from the code, not from looks
Every guardrail and stair rail has one hard limit: a sphere of 100 mm / 4 in must not pass through any opening. Measure the clear distance between finished baluster faces, not between centers. The Baluster spacing applies this limit directly — enter your code gap and it spaces everything equally below it.
How the baluster count is derived
For a run L, baluster width w and maximum gap g, the minimum count is ⌈(L − g) / (w + g)⌉. Intuition: each baluster plus one gap consumes w + g of the run, and the last gap may already count toward the limit. The calculator then distributes the leftover space into n + 1 equal gaps, including the two end gaps.
Mark edges, not centers
Set a stop block or measure from one post and mark the left edge of every baluster, exactly as the layout diagram shows. Marking centers and then dividing by baluster width is the classic drift error — by the far post you are a full baluster off.
Three mistakes to avoid
First, spacing between centers instead of clear faces. Second, ignoring the end gaps — they must satisfy the same code limit. Third, using the run between post centers instead of the clear run between post faces; subtract post width and handrail overhangs first.
Check before you cut
Lay the positions out with tape on the deck, place a sphere template (a 4 in hole cut in plywood) against the gaps, and only then cut. Ripping one baluster re-spreads every gap — the same way a Linear cutting calculator plan re-spreads every cut when one piece changes.