Space balusters (spindles) evenly and stay within the 4-inch rule. Enter the rail length between posts, the baluster width and the largest gap allowed to get how many balusters you need, the exact gap and on-center spacing, and a list of layout marks.
Last updated:
Units
in
Inside face to inside face of the posts
cm
Inside face to inside face of the posts
in
2×2 wood = 1.5 in; square metal 0.75 in
cm
in
Most codes: a 4 in sphere must not pass between balusters
cm
4 in ≈ 10.2 cm; many codes use 10 cm
Enter your values to see the result.
Advertisement
Advertisement
How to use the Baluster Spacing Calculator
Measure the rail length between the inside faces of the posts.
Enter the baluster width (1.5 in for 2×2 wood, 0.75 in for most metal).
Set the maximum gap: 4 in under most codes, or a little less to be safe.
Read the number of balusters and the exact gap, then cut a spacer block to that gap or use the layout marks.
Baluster spacing formula
Number of balusters: n = ⌊(L − g) ÷ (w + g)⌋ + 1 (the fewest that keep every gap under g)
Exact gap: gap = (L − n × w) ÷ (n + 1)
On-center spacing = gap + w
L is the rail length between posts, w the baluster width and g the gap limit. Example: 72 in rail, 1.5 in balusters, gaps under 4 in: n = ⌊68 ÷ 5.5⌋ + 1 = ⌊12.36⌋ + 1 = 13 balusters, gap = (72 − 19.5) ÷ 14 = 3.75 in, on center 5.25 in.
Balusters per section (gaps under 4 in)
Rail length
1.5 in wood (count / gap)
0.75 in metal (count / gap)
36 in
6 / 3 7/8 in
7 / 3 7/8 in
48 in
9 / 3 7/16 in
10 / 3 11/16 in
60 in
11 / 3 5/8 in
12 / 3 15/16 in
72 in
13 / 3 3/4 in
15 / 3 13/16 in
84 in
15 / 3 7/8 in
17 / 3 15/16 in
96 in
17 / 3 15/16 in
20 / 3 7/8 in
Installation tips
Cut a spacer block to the exact gap and use it between each baluster instead of measuring every time.
Start with a baluster centered in the section if you prefer, but then recheck the end gaps.
Measure each section separately: posts are rarely exactly the same distance apart.
Frequently asked questions
How do you calculate baluster spacing?
Add the baluster width to the maximum gap, divide the rail length minus one gap by that sum, take the whole-number part and add 1 so every gap stays under the limit: n = ⌊(L − g) ÷ (w + g)⌋ + 1. Then the exact gap is (L − n × w) ÷ (n + 1), so the spaces next to the posts match the spaces between balusters.
What is the 4-inch baluster rule?
The International Residential Code requires that a 4-inch diameter sphere cannot pass through any opening in a guard, which in practice means gaps of less than 4 inches between balusters (the triangle under stair rails allows 6 inches). Check your local code, and aim for about 3.5–3.75 in because wood can shrink and bow.
How many balusters do I need for an 8 ft section?
For about 93 in between posts with 1.5 in wide balusters and a 4 in maximum gap you need 17 balusters with gaps of 3 3/4 in. With 0.75 in metal balusters you need 19 balusters with gaps of about 3 15/16 in.
How many balusters per foot?
With 2×2 wood balusters and gaps just under 4 in, plan on roughly 2.2 balusters per linear foot of railing. With 3/4 in metal balusters it is about 2.5 per foot. Use the calculator for the exact count per section.
Should the gap at the posts be the same as between balusters?
Yes, for an even look the first and last gaps (next to the posts) should equal the gaps between balusters. The formula divides the leftover space into n + 1 equal gaps, which does exactly that.
Does this work for stair railings?
For stairs, the 4-inch rule is measured perpendicular to the balusters, so use the horizontal distance between posts with the same width and gap; the layout along the rake will be slightly longer. Many builders use two or three balusters per tread instead.
Is my data sent anywhere?
No. Everything is calculated in your browser.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.