Baluster Spacing Calculator (4-Inch Rule)
Enter the distance between posts, the baluster width and the maximum gap.
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Baluster Spacing Calculator (4-Inch Rule) by TheCalcTools
How the Baluster Spacing Calculator (4-Inch Rule) works
The International Residential Code (R312) requires guard openings that a 4-inch sphere cannot pass through. Choose the fewest balusters that keep every gap at or below your maximum.
n = ⌈(span − gap) ÷ (baluster width + gap)⌉, and the even gap is (span − n × width) ÷ (n + 1). Mark from the centre for a symmetrical layout.
The formula
Worked example
Using the default values shown in the calculator:
| Space between posts (in) | 72 |
|---|---|
| Baluster width (in) | 1.5 |
| Maximum gap (in) | 3.875 |
- 72 in between posts with 1.5 in balusters: 13 balusters with 3.750 in gaps.
Result:
| Balusters needed | 13 |
|---|---|
| Gap between balusters (in) | 3.750 |
| Code check | Gaps are under 4 inches |
| Centre-to-centre spacing (in) | 5.250 |
Frequently asked questions
Why 3⅞ inches and not 4?
Wood moves and balusters flex, so builders aim slightly under 4 inches to pass inspection.
Sources and further reading
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