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Baluster spacing calculator

How many, the gap, and a mark for each.

Open the Baluster Spacing tool

Free. Runs in your browser, no account, and keeps working offline.

What it does

Enter the clear length between the posts, the width of your balusters and the largest gap you will allow. The calculator finds the fewest balusters that keep every gap under that limit, spaces them evenly with a gap at each post, and lists where each one goes, measured from the left post.

Most residential guard rules say a 4″ sphere must not pass through, so the gap you enter should be a little under 4″ to leave room for a baluster that is slightly out of plumb or a rail that settles.

On a stair the balusters stand plumb, so the gap that matters is level, not along the slope. Give the rail angle, or lay out tread by tread with the rise and run, and the marks are carried back along the rail so you can pull them with a tape on the rail itself.

Worked example

A level rail with 72″ clear between the posts, 1 1/2″ square balusters, and every gap held under 3 3/4″ (so a 4″ sphere can't pass).

Balusters
13
Gap (each side of every baluster)
3 3/4″
On center
5 1/4″
First baluster's left edge from the post
3 3/4″

Mark the first edge, then step the on-center distance down the rail.

Try it with your numbers

Questions

How do you calculate baluster spacing?

Divide the clear space (rail length minus the total width of the balusters) by the number of gaps. With a gap at each post there is one more gap than balusters. The calculator picks the fewest balusters that keep that gap under your limit.

What is the maximum gap between balusters?

Residential codes commonly use a 4″ sphere rule: a 4″ ball must not pass through the guard. Your local code and inspector have the final word, so check before you drill.

Does it work on stair rails?

Yes. On a stair the gap is measured level because balusters stand plumb. Enter the rail angle, or the rise and run of the treads, and the marks come out along the rail.

Where do I start the layout?

Start from the left post. The first mark is the first baluster's left edge; step the on-center distance from there. Every gap, including the two at the posts, comes out the same.

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