Reference chart
Baluster Spacing Chart & Calculator
Balusters and the exact equal gap for each rail section length, at 1½-inch and ¾-inch balusters, every gap under the 4-inch sphere rule. The calculator below takes any length to the eighth of an inch and lists the centre marks.
Balusters and gap by rail section length
| Section | 1½″ balusters | Gap | On centre | ¾″ balusters | Gap | On centre |
|---|---|---|---|---|---|---|
| 36″ (3′ 0″) | 6 | 3.86″ | 5.36″ | 7 | 3.84″ | 4.59″ |
| 42″ (3′ 6″) | 8 | 3.33″ | 4.83″ | 9 | 3.53″ | 4.28″ |
| 48″ (4′ 0″) | 9 | 3.45″ | 4.95″ | 10 | 3.68″ | 4.43″ |
| 54″ (4′ 6″) | 10 | 3.55″ | 5.05″ | 11 | 3.81″ | 4.56″ |
| 60″ (5′ 0″) | 11 | 3.63″ | 5.13″ | 13 | 3.59″ | 4.34″ |
| 66″ (5′ 6″) | 12 | 3.69″ | 5.19″ | 14 | 3.7″ | 4.45″ |
| 72″ (6′ 0″) | 13 | 3.75″ | 5.25″ | 15 | 3.8″ | 4.55″ |
| 78″ (6′ 6″) | 14 | 3.8″ | 5.3″ | 17 | 3.63″ | 4.38″ |
| 84″ (7′ 0″) | 15 | 3.84″ | 5.34″ | 18 | 3.71″ | 4.46″ |
| 90″ (7′ 6″) | 17 | 3.58″ | 5.08″ | 19 | 3.79″ | 4.54″ |
| 96″ (8′ 0″) | 18 | 3.63″ | 5.13″ | 20 | 3.86″ | 4.61″ |
| 102″ (8′ 6″) | 19 | 3.68″ | 5.18″ | 22 | 3.72″ | 4.47″ |
| 108″ (9′ 0″) | 20 | 3.71″ | 5.21″ | 23 | 3.78″ | 4.53″ |
| 120″ (10′ 0″) | 22 | 3.78″ | 5.28″ | 26 | 3.72″ | 4.47″ |
Your numbers
code is under 4″; 3⅞″ leaves margin
Balusters
13
for 72″ between posts
Gap
3.75″
passes the 4″ sphere
On centre
5.25″
baluster + gap
First centre
4.5″
from the post face
Centre marks from the post: 4.5 · 9.75 · 15 · 20.25 · 25.5 · 30.75 · 36 · 41.25 · 46.5 · 51.75 · 57 · 62.25 · 67.5
n = ceil((L − max gap) ÷ (width + max gap)); gap = (L − n × width) ÷ (n + 1). Equal gaps, including the ones at each post.
Questions
How far apart can balusters be?
Close enough that a 4-inch sphere cannot pass through: IRC R312.1.3. In practice that means gaps of 3⅞ inches or less, and inspectors do test with a 4-inch ball. The chart and calculator keep every gap, including the ones next to the posts, at or under 3⅞.
How do I calculate baluster spacing?
For a section L inches long with balusters w inches wide, the smallest count that passes is n = ceil((L − 3.875) ÷ (w + 3.875)), and the equal gap is (L − n × w) ÷ (n + 1). A 72-inch section with 1½-inch balusters needs 13 balusters at 3¾-inch gaps. Centre-to-centre is gap + width, 5¼ here.
How do I lay them out on the rail?
Mark the first centre at gap + half a baluster from the post face, then every centre-to-centre distance. The calculator prints the marks. Cut a spacer block to the gap and you never measure again.
How many balusters for a deck?
Sum the sections. Each run between posts is its own calculation because the gap has to work out for that exact length; three 72-inch sections need 39, but a 70-inch and two 74s need 13 + 14 + 14 = 41. Buy 5% extra for splits and mis-cuts.
Does it apply to stairs?
Yes, with the sphere measured on the stair's diagonal, and a 6-inch sphere allowed in the triangle at the tread and riser. Use the stringer length between posts as the section length, and space along the slope.
Charts are computed from the formulas and code figures stated on the page (International Residential Code limits where cited). They are planning and take-off numbers, not engineering: species, grade, span tables and local amendments govern anything structural. Confirm with your inspector before you cut.