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Count balusters needed for code-compliant railing.
With these inputs:
| Result | Value |
|---|---|
| Balusters required | 120 balusters |
| Material cost | $540 |
| Spacing | 4" OC |
IRC requires gaps that won't pass a 4" sphere. Use ≤4.25" OC with 1.5" balusters.
Baluster spacing is governed by one rule that has not changed in decades: a four-inch sphere must not pass through any opening in the guard. That fixes your maximum gap and, with the baluster width, fixes the count. This works out how many balusters each railing section needs and the exact spacing to lay out, so every bay comes out even.
No more than four inches of clear space between them — the rule is that a four-inch sphere must not pass through. With 1 1/2 inch square balusters that means a repeat of about 5 1/2 inches on centre; with 3/4 inch round metal balusters, about 4 3/4 inches. Lay out from the exact clear gap, not from the centres, because the centre figure changes with baluster thickness.
It comes from child safety research: a four-inch sphere approximates the size below which a small child's head can pass through and become trapped. The same limit appears in the triangular opening at the bottom of a stair guard, where the figure is six inches, and in the gap between the bottom rail and the deck surface.
Take each railing section's clear length between posts, divide by the baluster width plus the maximum gap, and round up — then recalculate the actual gap so it comes out even across the bay. A 6 ft section between posts with 1 1/2 inch balusters needs about 13. Across 40 linear feet of railing, expect roughly 85 to 95.
Vertical is the norm and the safest bet. Horizontal rails are permitted in some jurisdictions and banned in others, because they form a ladder a child can climb — where allowed, they still have to meet the same four-inch rule. Cable rail is the horizontal exception most codes do accept, provided the cables are tensioned enough that the gap cannot be pushed past four inches.
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