Deck Railing & Baluster Calculator

Enter the total railing length. Balusters are spaced so the clear gap never exceeds your maximum.

Important: Guard height, strength and opening limits are code requirements. Verify post attachment and guard loads with the applicable code.

Tapeline — Deck Railing & Baluster Calculator

Printed September 29, 2026

Total railing length40 ft
Maximum post spacing6 ft
Post width3.5 in
Baluster width1.5 in
Maximum clear gap3.875 in

Results update as you type.

Code requires that a 4 in sphere cannot pass. 3⅞ in leaves a margin.

How it’s calculated

  • Sections = length ÷ max post spacing, rounded up; posts = sections + 1
  • Clear section = (length − posts × post width) ÷ sections
  • Balusters per section = (clear − gap) ÷ (baluster + gap), rounded up
  • Actual gap = (clear − balusters × width) ÷ (balusters + 1)

Worked example

40 ft of railing, posts at most 6 ft apart, 4×4 posts, 1½ in balusters.

Inputs

Total railing length
40 ft
Maximum post spacing
6 ft
Post width
3.5 in
Baluster width
1.5 in
Maximum clear gap
3.875 in

Results

Balusters
84
Posts
8
Sections
7
Balusters per section
12
Actual gap
3.58 in

Load this example into the calculator

Assumptions and limits

  • Straight run with equal sections. Corners and stair rails change the count.

Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.

Questions

What is the 4 inch rule for balusters?

Residential codes generally require that a 4 in sphere cannot pass through guard openings. Keeping gaps a bit under 4 in allows for wood movement.

How many balusters per foot?

With 1½ in balusters and gaps just under 4 in, expect about 2.2 per foot of clear railing.