The 4-inch sphere rule for deck railing baluster spacing
No opening in a level deck guard may pass a 4-inch sphere. Solve for an even gap under 4″ per bay — about 5½″ on center for 1½″ balusters.
What this diagram shows
An elevation of one bay of deck railing — a post at each end, a top and bottom rail, and evenly spaced vertical balusters between them. A 4-inch sphere is shown blocked in a gap because no opening in a level guard may pass a 4-inch sphere, and one gap is dimensioned at under 4 inches. With 1½-inch balusters that even gap lands near 5½ inches on center, about two balusters per foot of railing.
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Free deck railing calculator: posts, balusters, rails, caps, and hold-downs from one length. IRC R312 baluster spacing and post loads. No signup.
