Baluster Spacing Calculator (how many balusters & equal gaps under 4 in)

Understanding the baluster spacing calculator — concept and principle

This calculator works out how many balusters (spindles) you need between two posts so that every gap is equal and smaller than a safety limit, usually 4 inches. Deck and stair builders use it to lay out railings that meet code and look even.

Fit under the 4 in gap: with the default inputs the result is 13 · 3.75 in gap (52 for 4 sections).

How it is calculated

Number of balusters n = ⌊(L − limit) ÷ (w + limit)⌋ + 1

Gap = (L − n × w) ÷ (n + 1)

Centre-to-centre spacing = gap + w

First baluster centre from the post = gap + w ÷ 2

L is the clear length between posts, w the baluster width, limit the largest gap allowed.

The principle

With n balusters between two posts there are n + 1 gaps. The total length is n balusters plus n + 1 gaps, so solving for the gap gives the formula. The calculator picks the smallest n that makes the gap strictly less than the limit. The US International Residential Code (R312.1.3) requires that a 4-inch sphere cannot pass through a required guard, which protects small children from slipping through.

Worked example

Default inputs: 72 in between posts, 1.5 in balusters, 4 in limit, 4 sections.

n = ⌊(72 − 4) ÷ (1.5 + 4)⌋ + 1 = ⌊12.36⌋ + 1 = 13 per section.
Gap = (72 − 13 × 1.5) ÷ 14 = 52.5 ÷ 14 = 3.75 in (95 mm).
Centre-to-centre = 3.75 + 1.5 = 5.25 in; first centre = 3.75 + 0.75 = 4.5 in from the post.
Total = 13 × 4 = 52 balusters.

Each point is the calculator re-run with only “Clear length between posts (in)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Clear length between posts (in)	Balusters per section
18	3
36	6
54	10
72	13
90	16
108	19
144	26

How to read the result

Where does your result fall?

Mark the first centre from one post, then step along at the centre-to-centre spacing; the last gap should match. Check the gap with a 4-inch block. Different rules apply to the triangle under a stair rail and in other countries.

Common mistakes

Measuring post centre to post centre instead of the clear length between posts.
Making the gap exactly 4 in; it must be less.
Forgetting that square balusters turned diagonally take more space.

Keep in mind: Your local building code decides guard height and opening limits. Structural railings need code compliance and, where required, a qualified professional.

Frequently asked questions

What is the maximum baluster gap?

Under the US IRC, a 4-inch sphere must not pass, so gaps must be under 4 in.

How many balusters per foot?

With 1.5 in balusters and gaps just under 4 in, roughly 2 per foot.

Should I start at a post or at the centre?

Either works if gaps are equal; centring a baluster or a gap can look better on short sections.

Sources: International Code Council, 2021 International Residential Code, Section R312.1.3 "Opening limitations"

Checked 9 Nov 2026. International Code Council, 2021 International Residential Code, Section R312.1.3 “Opening limitations” (required guards shall not allow passage of a 4-inch-diameter sphere); equal-spacing arithmetic.

Your numbers