How the Baluster Spacing & Guard Height Calculator works

On a level run the balusters and the gaps between them have to fill the distance between the two posts exactly. With n balusters there are n + 1 gaps, because there is a gap at each post as well as between the balusters. This tool makes all of them equal.

For 72 in between posts and balusters 1 1/2 in wide: 12 balusters would leave (72 − 12 × 1.5) ÷ 13 = 4.1538 in, which is too wide. 13 leave (72 − 13 × 1.5) ÷ 14 = 3.75 in, and the balusters sit 5.25 in centre to centre.

Exactly at the limit is a fail

The code does not give a maximum gap. It says the guard must not have openings that allow a sphere of a stated diameter to pass. A 4 in ball just goes through a 4 in gap, so this tool takes the stricter reading: a gap equal to the sphere fails, and the code check passes only when every gap is less than the sphere. With 48 in between posts, 8 balusters leave gaps of exactly 4 in, so the tool uses 9, which leave 3.45 in. One of the calculators listed under Sources accepts a gap equal to its maximum; this tool does not.

Why the tool sometimes uses one more than the fewest

The exact gap is rarely a sixteenth, and a tape is read to the nearest 1/16 in. Each mark can therefore be 1/32 in out, and the gap between two marks can come out up to 1/16 in wider than the figure on paper. So the tool will not call a layout good unless the exact gap is at least 1/16 in under the sphere, and it also works out the gaps you would get from the rounded marks, by either column of the table, and holds those under the sphere. Where the fewest balusters on paper would leave less than 1/16 in to spare, it uses one more. That is the case for about one section in four. With 47 3/4 in between posts, 8 balusters give 3.9722 in on paper, but the nearest tape mark puts a gap at 4 in, so the tool starts from 9.

This margin is a choice made in this tool, not a code rule. If you set the count yourself and the gap is under the sphere by less than 1/16 in, the result is shown as a warning, not as a code failure, and both figures are on screen.

Marks, not a stepped-off gap

The mark-out table gives the distance of every baluster from the inside face of one post. Each figure is worked out from the post with the exact gap and then rounded once to the nearest 1/16 in, so an error in one mark is not carried into the next. Stepping a rounded gap along the rail does the opposite: all the rounding ends up in the last gap. With 68 1/2 in between posts and 12 balusters the exact gap is 3.8846 in; a spacer block cut to 3 7/8 in and stepped off from one post leaves the last gap at 4 in.

Stairs are not laid out

The layout is for level runs only. On the open side of a stair the balusters stand plumb between rails that slope, and the gap between two of them depends on the exact slope of the stair, so a small error in measuring the slope changes the gap. This tool does not lay those out. The code tests the openings of a guard on the open side of a stair with a 4 3/8 in sphere (R312.1.3 Exception 2), and the triangle formed by the riser, the tread and the bottom rail with a 6 in sphere (Exception 1). A landing is a level run. The guard height check does cover a stair guard: there the height is measured plumb from the line through the tread nosings, not from a tread.

The code limits

The figures are those of Section R312.1 of the 2021 IRC, in this tool's words. They apply to guards the code requires.

A drop of exactly 30 in does not require a guard, because the wording is "more than". A guard of exactly 36 in passes, because the wording is "not less than". The drop is the largest one found within 36 in out from the open edge, so ground that slopes away from a deck can require a guard on a deck that is low at the edge itself.

Editions

The 2018 and 2024 texts were read beside the 2021 text. In these three subsections the 2018 edition gives the same figures; the 2021 edition adds floors to the walking surfaces it names. The 2024 edition moves the section from R312 to R321 and reads the same. States and cities amend this section, some to a taller guard or a smaller opening: the edition and amendments your jurisdiction has adopted are what govern.

What this tool does not check

• Strength. Table R301.5 asks a guard to take a 200 lb load at any point along its top, and infill such as balusters to take 50 lb spread over one square foot. How the posts, rails and balusters are fixed decides that.
• The opening under the bottom rail. The opening limit runs from the walking surface up, so the space between the deck and the bottom rail is under the same sphere test.
• Balusters on the open side of a stair, and guards on ramps, which R312.1.1 also names.
• Openings that are not between plumb balusters: horizontal rails, cable, glass or mesh infill, and decorative panels.
• Post spacing and size, handrail shape and grip, and guards in buildings covered by the International Building Code.
• What the guard is made of. Insect screening does not count as a guard (R312.1.1), and exterior plastic composite guards have their own section (R312.1.4).
• Baluster width that varies. Enter the thinnest width, measured, because a thinner baluster leaves a wider gap.

This tool cannot be published unless it reproduces every example below. 3 are copied from a published source. 15 were worked by hand from the cited rule.