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Baluster spacing calculator: how many balusters and how far apart?

Calculate the number of balusters per railing section and the exact spacing that keeps every gap under the code limit.

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Balusters per section

13

Gap between balusters

3.75 in

Total balusters

52

On-center spacing
5.25 in
To buy with 5% spare
55

The 4-inch rule

Most US building codes, based on the International Residential Code (IRC), require guards whose openings do not let a 4-inch sphere pass through. The rule protects small children, whose heads can slip through larger gaps. On stairs, the triangle formed by the tread, riser and bottom rail may allow a 6-inch sphere, and the guard on the open side of a stair may allow 4 3/8 inches.

How to calculate the number of balusters

For a section of length L between posts, with baluster width b and maximum gap g:

  • Balusters per section: n = (L − g) ÷ (b + g), rounded up
  • Actual gap: (L − n × b) ÷ (n + 1)
  • On-center spacing: actual gap + b

Worked example

A deck has 4 sections of 72 inches between posts. You use 1.5-inch wood balusters (nominal 2 × 2) and aim for gaps under 3 7/8 inches.

  1. Balusters per section: (72 − 3.875) ÷ (1.5 + 3.875) = 12.7, rounded up to 13.
  2. Actual gap: (72 − 13 × 1.5) ÷ 14 = 3.75 inches.
  3. On-center spacing: 3.75 + 1.5 = 5.25 inches.
  4. Total: 13 × 4 = 52 balusters, or 55 with 5% spare for splits and bad pieces.

Common baluster widths

Baluster Actual width Per 6-ft section (gap under 3 7/8 in)
Round metal 0.5 to 0.75 in 15 to 16
Square metal 0.75 in 15
Wood 2 × 2 1.5 in 13
Turned wood 1.75 in 13

Railing heights

  • A guard is required where a deck, porch or balcony is more than 30 inches above the ground below.
  • Residential guards must be at least 36 inches high under the IRC; many towns and all commercial buildings require 42 inches.
  • Stair handrails are typically 34 to 38 inches above the tread nosing.

Installation tips

  • Cut a spacer block to the exact gap and use it between each baluster.
  • Start from the center of the section if you want equal gaps at both posts, or from one post when the math comes out even.
  • Check for plumb every few balusters with a level.
  • Wood shrinks as it dries; pressure-treated balusters installed wet can open up, so leave a small margin under the limit.
  • Codes vary by town, so confirm with your local building department before you build.

Frequently asked questions

What is the maximum baluster spacing?

Codes based on the International Residential Code do not allow a 4-inch sphere to pass between balusters, so the gap must be under 4 inches. Many builders aim for about 3 3/4 inches to allow for wood movement.

How many balusters do I need for a 6-foot section?

With 1.5-inch balusters and gaps under 3 7/8 inches, a 72-inch section needs 13 balusters, which leaves 14 gaps of 3.75 inches.

How do I space balusters evenly?

Subtract the total width of the balusters from the section length, then divide by the number of gaps, which is one more than the number of balusters. Mark the first gap from a post and use a spacer block cut to that width.

How tall must a deck railing be?

Under the IRC, a guard is required where the deck is more than 30 inches above grade, and it must be at least 36 inches high. Commercial buildings and some local codes require 42 inches. Check with your local building department.

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Results are estimates provided for information purposes. They do not replace professional advice.

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