Deck Railing Spacing Calculator — How Many Balusters Do I Need?
Enter a railing section length to get the baluster count and the exact on-center spacing, code-friendly at 4 in or 6 in. No sign-up, no data stored.
Baluster plan
How baluster spacing is calculated
- Balusters per section = (section length − baluster width) ÷ (gap + baluster width), rounded up. For 120 in, 1.5-in balusters at a 4-in gap: (120 − 1.5) ÷ 5.5 ≈ 21.5 → 22.
- Actual gap = (section length − balusters × width) ÷ (balusters + 1). This is the real open space between balusters — it must stay at or under your max gap.
- On-center spacing = section length ÷ (balusters + 1) — the number you use for layout marks.
- Total = per-section count × number of sections (add a few spares for cuts and mistakes).
The IRC rule is simple: no gap larger than 4 inches anywhere in the guard. When in doubt, layout at 4 in on center with 1.5-in balusters — that always passes and looks clean.
Section length → baluster count reference
Balusters needed at 1.5-in width and a 4-in gap (IRC standard).
| Section length | Balusters | Actual gap | On center |
|---|---|---|---|
| 4 ft (48 in) | 9 | 3.9 in | 4.8 in |
| 6 ft (72 in) | 13 | 3.9 in | 5.1 in |
| 8 ft (96 in) | 18 | 3.8 in | 5.1 in |
| 10 ft (120 in) | 22 | 3.8 in | 5.2 in |
| 12 ft (144 in) | 26 | 3.9 in | 5.3 in |
Last updated: October 2026 · Estimates assume 1.5-in balusters and a 4-in max gap. Verify your local code — height and spacing rules vary by jurisdiction.
Deck railing spacing FAQ
The modern IRC requires 4 inches maximum for residential decks. 6 inches survives only in older or amended local codes — if an inspector or buyer is checking, 4 in is the safe choice.
Measure the clear span between the inside faces of the posts. If the rail ends at a wall or newel post, measure to that surface. Don't include post widths.
36 inches minimum for decks over 30 in above grade (IRC); some municipalities want 42 in. Check local code before you build, and measure from the deck surface to the top of the rail.
No — horizontal rails in the open area are a climbing hazard for kids and generally banned or restricted. Use vertical balusters or approved infill.
Add up every section, then add 5–10% for waste. For a typical 20-ft deck split into two 10-ft sections that's 44 balusters, plus 3–4 spares.
Related calculators & guides
Deck Railing Height & Spacing Code Guide
Guard height, baluster gaps and the IRC rules that decide whether your deck passes inspection.
Read the guide →