Enter your rail length, post dimensions, and baluster size to get the exact number of balusters and code-compliant spacing. Works for deck railings, staircases, and porch guards. Free — no signup required.
Typical baluster count for common rail lengths — 2 posts, 1.5″ square balusters, 3.5″ max gap.
| Rail Length | Balusters | Approx. Gap |
|---|---|---|
| 6 ft | 14 | ~3.5″ |
| 8 ft | 19 | ~3.5″ |
| 10 ft | 23 | ~3.5″ |
| 12 ft | 28 | ~3.5″ |
| 16 ft | 37 | ~3.5″ |
A 12-foot deck rail section with two 4×4 posts (3.5″ each) and 1.5″ square balusters at a 4″ maximum gap: usable span = 144″ − (2 × 3.5″) = 137″. Dividing 137″ by 5.5″ (1.5 + 4.0) gives 24.9, so you need 25 balusters. Actual gap = (137 − 25 × 1.5) / 24 = (137 − 37.5) / 24 = 99.5 / 24 ≈ 4.15″. That exceeds 4″, so round up to 26 balusters: (137 − 26 × 1.5) / 25 = 97.5 / 25 = 3.9″ — code compliant.
IRC and most Canadian codes require a maximum 4-inch clear gap between balusters — small enough that a 4-inch sphere cannot pass through.
With two 4×4 posts and 1.5″ square balusters, a 10-foot rail typically needs 23–24 balusters at just under 4-inch spacing.
No. The 4-inch maximum is the clear open gap between balusters. Center-to-center spacing = baluster width + gap.
1.5×1.5 inch pressure-treated square balusters are the most common for wood decks. Metal and composite spindles are often 1 or 1.75 inches.
The same 4-inch maximum gap applies to stair guards. The balusters are installed plumb even on a raked rail.
Typical Results
💡 Always verify the final gap calculation before cutting — measure the usable span with your actual posts in place, not from the plan.
Common Mistakes