What is the 4 inch rule?

The IRC requires that a 4 in sphere can't pass through guard openings.

How do I lay out balusters?

Mark on-center spacing from the middle of the section out to the posts.

What about stair railings?

Measure horizontal clear length; stairs allow a 4⅜ in sphere in the IRC.

How high must a deck railing be?

36 in under the IRC for decks over 30 in above grade.

Vertical or horizontal infill?

Vertical if children are around — horizontal rails are climbable.

What if the spacing doesn't come out even?

Adjust the actual gap so every space is equal, including the ends.

Baluster spacing calculator

Clear distance between posts, baluster width and maximum gap: you get balusters per section, the actual gap and on-center spacing for layout.

Balusters per section

12 unit

Total balusters12 unit
Actual gap between balusters3.815 in
On-center spacing5.587 in

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How it's calculated (visible formula)

n = ceil((L − max gap) ÷ (baluster + max gap)); gap = (L − n × baluster) ÷ (n + 1)

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Measure the clear distance between posts (not center to center).
Enter baluster width.
Set the maximum gap (4 in under the IRC).
Work out how many balusters keep the gap under the limit.
Spread the actual gap and lay out from the center of the section.

Worked example: 71 in section, 1¾ in balusters, 4 in max gap

(71 − 4) ÷ 5.75 = 11.7 → 12 balusters
(71 − 12 × 1.75) ÷ 13 ≈ 3.85 in

→ 12 balusters with a 3.85 in gap, 5.6 in on center.

Common mistakes

Measuring post centers instead of the clear opening.
Leaving uneven gaps at the ends.
Forgetting the gap under the bottom rail.

Pro tips

Cut a spacer block to the actual gap for fast installation.
Guards must be 36 in high on decks (42 in commercial).
Avoid climbable horizontal rails where children play.
Fasten balusters top and bottom so they don't twist.

From calculation to quote

Add labor, overhead and your margin, and generate the professional PDF.

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FAQs

What is the 4 inch rule?: The IRC requires that a 4 in sphere can't pass through guard openings.
How do I lay out balusters?: Mark on-center spacing from the middle of the section out to the posts.
What about stair railings?: Measure horizontal clear length; stairs allow a 4⅜ in sphere in the IRC.
How high must a deck railing be?: 36 in under the IRC for decks over 30 in above grade.
Vertical or horizontal infill?: Vertical if children are around — horizontal rails are climbable.
What if the spacing doesn't come out even?: Adjust the actual gap so every space is equal, including the ends.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.