LayoutWise

Baluster Spacing Calculator

Enter the clear space between each pair of posts. You get the number of balusters, a gap that's identical every time and under the 4-inch rule, and the marks to lay them out.

Your railing

1. Sections between posts

Measure the clear space between post faces, not post centers.

Section 1
ft
in
Section 2
ft
in
Section 3
ft
in
2. Balusters

Balusters

37

3 sections

Gap (section 1)

3 3/8"

every gap equal

On center

4 7/8"

baluster to baluster

Rail length

15' 10 1/2"

top and bottom

Section 1

14 balusters, 3 3/8" gaps

Mark baluster centers from the left post face at: 4 1/8", 9 1/16", 13 15/16", 18 7/8", 23 3/4", 28 5/8", 33 9/16", 38 7/16", …

Section 2

14 balusters, 3 3/8" gaps

Section 3

9 balusters, 3 5/16" gaps

Shopping list

  • ✓Balusters: 37 plus 2 spares (1 1/2" wide, about 31" long)
  • ✓Top and bottom rails: 15' 10 1/2" each
  • ✓Fasteners: 148 screws (2 at top, 2 at bottom of each baluster)

Cut a spacer block to the gap width and use it between every baluster — it's faster and more accurate than measuring each one.

The 4-inch rule

Residential code (IRC R312.1.3) says a 4-inch sphere must not pass through any opening in a guard. In practice that means every gap between balusters — and between the end baluster and the post — must be under 4 inches. Many builders aim for about 3 1/2 inches so wood shrinkage never pushes a gap over.

How to space balusters evenly

With n balusters between two posts there are n + 1 gaps. So: number of balusters = (space − max gap) ÷ (baluster width + max gap), rounded up. Then the exact gap is (space − balusters x width) ÷ (balusters + 1).

Example: 72" between posts with 1 1/2" balusters and a 3 1/2" maximum gap: (72 − 3.5) ÷ 5 = 13.7, so 14 balusters. The gap is (72 − 21) ÷ 15 = 3.4". Every gap, including the two at the posts, is the same.

Guard height

Decks and stairs with more than a 30" drop need a guard at least 36" tall under the residential code; some towns require 42". Check locally before you buy balusters, since they're sold by length.

Need the stairs too? Try the stair calculator.

Sources