How far apart should balusters be?
Close enough that a 4 inch sphere cannot pass between them, which is the rule in section R312.1.3 of the International Residential Code. Between posts 72 in apart, wood 2×2 balusters (1.5 in) with gaps of at most 3.5 in need 14 balusters, with an even gap of 3 3/8 in and 4 7/8 in on center.
Worked example: a 72 in railing section
- Balusters: (72 − 3.5) ÷ (1.5 + 3.5) = 13.7, rounded up to 14.
- Wood used by the balusters: 14 × 1.5 = 21 in, which leaves 51 in of open space.
- There is one more gap than balusters: 51 ÷ 15 = 3.4 in per gap.
- To the nearest 1/16 in: 3 3/8 in, safely under 4 in.
- On center: 1.5 + 3.4 = 4.9 in, or 4 7/8 in.
Balusters per railing section
With gaps of at most 3.5 in, for wood 2×2 balusters (1.5 in) and slim metal balusters (0.75 in).
| Space between posts | Wood balusters | Wood gap | Metal balusters | Metal gap |
|---|---|---|---|---|
| 36 in | 7 | 3 3/16 in | 8 | 3 5/16 in |
| 48 in | 9 | 3 7/16 in | 11 | 3 5/16 in |
| 60 in | 12 | 3 1/4 in | 14 | 3 5/16 in |
| 72 in | 14 | 3 3/8 in | 17 | 3 5/16 in |
| 84 in | 17 | 3 1/4 in | 19 | 3 1/2 in |
| 96 in | 19 | 3 3/8 in | 22 | 3 7/16 in |
The 4 inch sphere rule
Section R312.1.3 of the International Residential Code says a required guard must not have openings that let a 4 inch sphere pass, from the walking surface up to the required guard height. On the open side of stairs the limit is 4 3/8 in. The gap under the bottom rail counts too.
- Guards are generally required where a deck, porch or landing is more than 30 in above the grade below.
- Wood balusters can shrink and twist, so aim for about 3.5 in rather than just under 4.
- Your local building department may have amended the code. Check before you build.
How the calculator works
Balusters = (space − largest gap) ÷ (baluster width + largest gap), rounded up
Even gap = (space − balusters × baluster width) ÷ (balusters + 1)
On center = baluster width + even gap
For level railings. On a stair railing the balusters follow the slope, so measure the clear space between posts along the rail and check the 4 3/8 in limit.
Frequently asked questions
What is the maximum baluster spacing?
Under IRC R312.1.3, a 4 inch sphere must not pass through the guard, so the clear gap must be less than 4 in. On the open side of stairs the limit is 4 3/8 in.
How many balusters do I need for a 6 ft railing?
For 72 in between posts with wood 2×2 balusters and gaps of at most 3.5 in, 14 balusters.
Is baluster spacing measured on center?
No. The 4 in rule is about the clear opening between balusters. On-center spacing is only a handy way to mark the rail.
How many balusters per foot?
About 2.3 wood 2×2 balusters per foot of space between posts at a gap of about 3.5 in, or about 2.8 slim metal ones.
Sources
- City of Albany, Georgia, Residential guard requirements (IRC R312.1.3)
- JLC, Code-Compliant Guardrails