What is the 4-inch rule for balusters?

IRC R312.1.3 says a guard must not have any opening through which a 4" sphere can pass. It is about the sphere, not the nominal gap, so a gap measured at exactly four inches fails — aim for 3 3/4" or less and you have room for the wood to move and for your saw to be human. It applies to guards: deck rails, landings, stairs. A privacy screen or a slat wall is not a guard and the rule does not apply.

How do I space balusters evenly?

You cannot pick the count and the gap independently — the span is fixed, so choosing one determines the other. Decide the gap you want, work out how many slats that implies, round to a whole number, and then recompute the gap that actually results. That last step is the one people skip, and it is why the last gap so often comes out different from the rest.

Should the run start with a gap or a slat?

A gap, for balusters between posts — a baluster hard against a newel looks like a mistake and leaves nowhere to fasten it. A slat, for a slat wall or a privacy screen, where an incomplete slat at one end is the thing that reads as unplanned. The arithmetic differs: n slats with gaps at both ends make n+1 gaps, while n slats flush to the ends make only n−1.

How do I mark the layout without the spacing drifting?

Measure every mark from the same end of the span, never from the slat before it. A gap of 3 13/16" measured twenty times in a row accumulates every sixteenth you rounded, and by the far post you are half an inch out with nowhere to put it. Hook the tape once, mark all the positions, then cut.

What if the opening isn't the same at the top and the bottom?

Measure it in both places and lay out to the smaller reading. Posts lean, and a bay that is 72" at the rail is often a quarter inch tighter at the shoe. Laying out to the larger number means the last slat runs out of room; laying out to the smaller one means the end gaps close up very slightly, which nobody will ever see.

Baluster spacing and printable template

Even spacing across any span, checked against the guard rule, with every mark measured from the same end so nothing drifts by the far post. Print the gap itself as a true-size spacer below.

Span to fill

Post face to post face

Slat width

= 1.5"

Gap you want

= 3.75"

At the ends

A gap against each post. Standard for balusters between newels.

Advanced settings32" slat height (drawing only)

Slat height

Drawing only

13 slats across 72"

3 3/4"gap

Slats

Gaps

Slat material

1.6 ft

Marks from the left end

Slat 1: 3 3/4" → 5 1/4"
Slat 2: 9" → 10 1/2"
Slat 3: 14 1/4" → 15 3/4"
Slat 4: 19 1/2" → 21"
Slat 5: 24 3/4" → 26 1/4"
Slat 6: 30" → 31 1/2"
Slat 7: 35 1/4" → 36 3/4"
Slat 8: 40 1/2" → 42"
Slat 9: 45 3/4" → 47 1/4"
Slat 10: 51" → 52 1/2"
Slat 11: 56 1/4" → 57 3/4"
Slat 12: 61 1/2" → 63"
Slat 13: 66 3/4" → 68 1/4"

Every one of these is measured from the same end of the span, not from the slat before it. Hook the tape once and mark them all — that is what keeps a sixteenth of error from becoming half an inch by the far post.

Other counts

"Passes" here means the 2021 IRC's 4" sphere rule for a residential guard, which is a default rather than law where you are building — your jurisdiction amends it and commercial work answers to a different code. Ring your building department before you cut, not after.

Gap gauge, at true size

Not the whole run — the marks above already give every position from one end of the tape. What's worth printing is the gap itself, as a spacer you butt against the last slat to set the next one, faster than measuring each mark by hand. Print at 100%, check the ruler, then cut a scrap to this length.

Check this first

Put a tape on the printed ruler. If it does not read exactly 6 inches, the print was scaled — set the print dialog to 100% or “Actual size” and print it again. Do not cut wood from a template that failed this.

One gap, spacer length

Cut a scrap block to this length and use it to space each slat off the last one — faster than measuring every mark, and it keeps every gap the same as the one before it.

Before you cut anything

Measure the opening at the top and at the bottom. Posts lean, and a bay that is 72" at the rail is often 71 3/4" at the shoe — lay out to the smaller reading and the last gap closes up rather than running out.

The gap here is exact to the thousandth and your saw is not. Cut one slat, hold it in place, and check the first gap against the mark before you cut the other nineteen.

Where to buy this

13 slats — 1.6 ft of materialHome Depot Lowe's

The same spacing maths fits wood, composite and metal balusters, and a slatted screen. What you are buying is your call; the count is not.

Plain search links, nothing sponsored. Prices and stock are the shop's, and a local lumberyard will usually beat both on solid stock.

Worked example: a 72" railing

Between posts 72" apart, 13 balusters 1 1/2" wide leave 14 equal gaps of 3 3/4", with a gap at each post.

That passes the 2021 IRC rule that a 4" sphere must not pass through a guard.

Mark the first baluster's edge 3 3/4" from the post and every one after it 5 1/4" further on.

Balusters: 13
Gap: 3 3/4"
Gaps: 14
First mark: 3 3/4"

These are the figures the calculator above opens on. Change any field and it works the same problem for your numbers.

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About spacing balusters and slats

What is the 4-inch rule for balusters?: IRC R312.1.3 says a guard must not have any opening through which a 4" sphere can pass. It is about the sphere, not the nominal gap, so a gap measured at exactly four inches fails — aim for 3 3/4" or less and you have room for the wood to move and for your saw to be human. It applies to guards: deck rails, landings, stairs. A privacy screen or a slat wall is not a guard and the rule does not apply.
How do I space balusters evenly?: You cannot pick the count and the gap independently — the span is fixed, so choosing one determines the other. Decide the gap you want, work out how many slats that implies, round to a whole number, and then recompute the gap that actually results. That last step is the one people skip, and it is why the last gap so often comes out different from the rest.
Should the run start with a gap or a slat?: A gap, for balusters between posts — a baluster hard against a newel looks like a mistake and leaves nowhere to fasten it. A slat, for a slat wall or a privacy screen, where an incomplete slat at one end is the thing that reads as unplanned. The arithmetic differs: n slats with gaps at both ends make n+1 gaps, while n slats flush to the ends make only n−1.
How do I mark the layout without the spacing drifting?: Measure every mark from the same end of the span, never from the slat before it. A gap of 3 13/16" measured twenty times in a row accumulates every sixteenth you rounded, and by the far post you are half an inch out with nowhere to put it. Hook the tape once, mark all the positions, then cut.
What if the opening isn't the same at the top and the bottom?: Measure it in both places and lay out to the smaller reading. Posts lean, and a bay that is 72" at the rail is often a quarter inch tighter at the shoe. Laying out to the larger number means the last slat runs out of room; laying out to the smaller one means the end gaps close up very slightly, which nobody will ever see.