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Deck Railing & Baluster Spacing Calculator
Lay out your deck railing so the baluster gaps are even AND code-legal. Enter your railing length and post spacing and the calculator solves the baluster count and the exact even gap that keeps a 4-inch sphere out (IRC R312).
Balusters
91
13 per section
Even gap
3.26″
Passes (under 4″)
Railing layout
Rail sections7
Rail posts8
Clear span per section65.1″
Guard height (R312)36″ min
Even gap = (section clear, minus balusters times width) divided by (balusters plus 1). Code requires the gap to reject a 4″ sphere (IRC R312.1.3); 3½″ is the safe build target. Guards are 36″ min on residential decks.
Deck Railing & Baluster: what to know
- •The code test is a 4" sphere: the clear gap between balusters (and between a baluster and a post) must be under 4". Build to 3½" so you're never on the line at inspection.
- •Residential deck guards must be at least 36" tall, measured from the deck surface to the top of the rail.
- •Even spacing = (section clear width − total baluster width) ÷ (number of gaps). The calculator does this per section so every bay matches.
Want a permit-ready deck plan?
This estimate is built for planning. For your permit packet, DeckHelm Pro turns these numbers into a code-referenced framing plan, footing schedule and material list as a printable PDF.
See Pro →