Enter the space between two posts, the width of your balusters and the gap you are aiming for. The calculator gives the number of balusters and the exact even gap, so the openings stay under the 4 in limit.
The 4 inch sphere rule
Residential codes require guard openings small enough that a 4 in sphere cannot pass through. The City of Kent, Washington’s guards handout, based on the International Residential Code, states that guards must not allow the passage of a sphere 4 in or more in diameter. The calculator adds a baluster if the gap would reach 4 in. Local rules can differ, so check yours.
Baluster formula
Balusters = (space − target gap) ÷ (baluster width + target gap), rounded up Actual gap = (space − balusters × width) ÷ (balusters + 1)
Worked example: 96 in between posts, 1.5 in balusters
- Balusters: (96 − 3.5) ÷ 5 = 18.5, so 19.
- Actual gap: (96 − 19 × 1.5) ÷ 20 = 3.375 in.
- Mark centers every 4.875 in from the first gap.
Balusters per section (1.5 in balusters, 3.5 in target)
| Space between posts | Balusters | Gap |
|---|---|---|
| 36 in | 7 | 3.19 in |
| 48 in | 9 | 3.45 in |
| 60 in | 12 | 3.23 in |
| 72 in | 14 | 3.4 in |
| 84 in | 17 | 3.25 in |
| 96 in | 19 | 3.38 in |
Questions
How many balusters for an 8 ft section?
19 balusters of 1.5 in with a 3.38 in gap.
Can the gap be exactly 4 inches?
No. The sphere must not pass, so keep the gap under 4 in; most builders aim for about 3½ in.
How do I lay them out?
Start with a gap at each post and mark the on-center spacing from there.
Sources
- Guards, City of Kent, WA Building Services.
Used for: Guards must not allow passage of a sphere 4 in or more in diameter.