How many balusters for a deck railing?
A 6-foot rail section in 2x2 balusters takes 14, spaced so no 4" sphere passes through. Counts by rail length, and the code rule that decides them.
A 6' clear span takes 14 1 1/2" balusters, at 3 3/8" between them.
That keeps every gap under the 4" sphere the IRC allows through a guard — with 5/8" to spare, which is room for the wood to move and for your saw to be human.
How that is worked out
- The rule is not a maximum gap, it is a sphere: IRC R312.1.3 says no opening in a guard may pass a 4" sphere. A gap measured at exactly four inches fails.
- 14 balusters across a 72" span leaves 15 gaps — one more than the baluster count, because there is a gap at each end as well as between.
- 72" less 21" of baluster, divided over 15 gaps, is 3 3/8" each.
1 1/2" balusters, gap at each end, aiming just under 4"
| Clear span | Balusters | Gap | Spare under the sphere |
|---|---|---|---|
| 36" (3 ft) | 7 | 3 3/16" | 13/16" |
| 48" (4 ft) | 9 | 3 7/16" | 9/16" |
| 60" (5 ft) | 11 | 3 5/8" | 3/8" |
| 72" (6 ft) | 14 | 3 3/8" | 5/8" |
| 84" (7 ft) | 16 | 3 1/2" | 1/2" |
| 96" (8 ft) | 19 | 3 3/8" | 5/8" |
| 120" (10 ft) | 23 | 3 9/16" | 7/16" |
Worth knowing
- IRC R312.1.3 and the 4" sphere are 2021 IRC — defaults, not law. Your jurisdiction amends them and the building department decides. Stair guards and the triangular opening at the bottom of a stair rail have their own rules.
- Aim under the limit rather than at it. Balusters move, posts lean, and an inspector's sphere does not care that it measured right on the day you built it. A quarter inch of margin costs nothing.
- Measure the clear span between posts, not the rail's overall length. Each section between posts is its own layout and they are rarely all the same.
- The rule applies to guards — deck rails, landings, stairs. A privacy screen or a slat wall is not a guard and it does not apply.
Work out your own numbers
- Baluster and slat spacing
Even gaps across any span, checked against the 4-inch guard rule, with tape-measure marks that don't drift.
- Stair stringers
Divide the rise into even risers, lay out the notches, and check the result against the code.
- Deck and fence takeoff
Boards, joists, posts, pickets, screws and bags of concrete, with the gaps between boards properly counted.
Related questions
- How far apart can deck balusters be?
- Less than 4" — and specifically, no gap may let a 4" sphere through. It is a sphere rule rather than a flat measurement, so a gap that measures exactly 4" fails. Most builders lay out for about 3 1/2" to leave room for wood movement and for the posts not being perfectly plumb.
- How many balusters do I need for a 6 foot railing?
- 14 in 2x2 stock, at about 3 3/8" between them. Work each section between posts separately: they are seldom all the same clear span, and a count that is right for one is a gap over the limit on the next.
- Why is it a 4 inch sphere and not a 4 inch gap?
- Because the rule exists to stop a small child's head passing through, and a head is not flat. A sphere tests the opening in every direction at once, which catches a gap that is narrow in one place and wide in another — a flat measurement would pass openings the sphere does not.