Baluster Calculator

Balusters n = ceil( ( L − g ) ÷ ( w + g ) ) then gap = ( L − n × w ) ÷ ( n + 1 )

L

Length of the run between the two end posts, in inches

w

Width of one baluster, in inches

g

Largest permitted clear gap, in inches (4 for the 4 inch sphere rule, 4.33 for 110 mm)

n

Number of balusters, rounded up

gap

The clear gap you actually get, in inches — the number the inspector measures

Use it once the two end posts are fixed, because the run is measured between them rather than between the outside faces of the railing — that is the mistake that costs a baluster at each end. Measure the run, decide the baluster (2 in square wood actually measures 1.5 in, and a nominal half-inch metal baluster often measures less), then set the limit your code or your customer requires: 4 in for the 4 inch sphere rule, 4.33 in for a 110 mm limit, 3.94 in for 100 mm. The formula rounds the count up, so the gap you get is always at or under the limit; if the page reports a gap of exactly the limit, that run happened to divide evenly and you should not shorten it. For a stair railing the run is measured along the rake, and the gap is still measured horizontally from baluster to baluster, so use the sloped length of the rail for the run and keep the same limit.

1. A 20 ft deck railing of 2 in square wood balusters, 4 in limit

   1. Run between the end posts: 20 ft × 12 = 240 in
   2. Balusters: (240 − 4) ÷ (1.5 + 4) = 236 ÷ 5.5 = 42.91, rounded up to 43
   3. Gap: (240 − 43 × 1.5) ÷ 44 = 175.5 ÷ 44 = 3.99 in, which is 101.3 mm
   4. Check: 3.99 in is under the 4 in limit, so the run is compliant

   These are the page defaults. The interesting part is the rounding: the bare arithmetic asks for 42.91 balusters, so a 43rd is added, and because a whole baluster is 1.5 in wide the gap drops from 4.00 to 3.99 in. Nothing is wasted — the extra baluster is what buys you the compliance margin, and the 101.3 mm it leaves is what a 110 mm specification would be measuring.

2. An 8 ft stair railing over a landing, 4 in limit

   1. Run between the end posts: 8 ft × 12 = 96 in
   2. Balusters: (96 − 4) ÷ 5.5 = 16.73, rounded up to 17
   3. Gap: (96 − 17 × 1.5) ÷ 18 = 70.5 ÷ 18 = 3.92 in, which is 99.5 mm

   Compare this with the 20 ft run above: the length falls by 60% and the count falls by 60% with it, while the gap barely moves — 3.92 in against 3.99 in. That is the character of this page. The count scales with the run; the gap is pinned by the limit, so it comes out somewhere between 3.5 and 4.0 in on almost every job, and a gap well below 4 in only means the run divided awkwardly.

3. A 12 ft metal railing held to a 110 mm limit

   1. Run between the end posts: 12 ft × 12 = 144 in
   2. Limit converted: 110 mm ÷ 25.4 = 4.331 in
   3. Balusters: (144 − 4.331) ÷ (0.5 + 4.331) = 139.67 ÷ 4.831 = 28.91, rounded up to 29
   4. Gap: (144 − 29 × 0.5) ÷ 30 = 129.5 ÷ 30 = 4.32 in, which is 109.6 mm

   The same run under the 4 inch sphere rule takes 32 balusters, so the two specifications are three balusters apart on a 12 ft railing — the 110 mm limit is the looser one, and a drawing that swaps between them without saying so will be built one way and inspected the other. Note the width as well: half-inch metal balusters are thinner than 2 in wood, so they need more of them to hold the same limit.

This page does the spacing arithmetic and nothing else. It does not know your code: the 4 in, 100 mm and 110 mm limits are inputs, the page has no idea which one applies to a deck railing, a stair railing or a commercial guard, and it will happily compute a non-compliant railing if you type a limit that is too large. It does not check the height of the guard, the load it must carry, the size of the rail, or how the balusters are fixed — a 2 in square baluster spaced at 4 in centers can still fail on how it is attached to the rail. The count assumes balusters sit between two end posts that already exist and are the same width as each other; if the end posts are thicker than the balusters, measure the run between their inner faces as the field asks. Only one baluster width is used for the whole run, and decorative patterns that alternate thick and thin, or a run with a gate in it, need to be split into two calculations. Finally, the arithmetic treats the run as straight: a curved rail is a series of chords, and each one is its own run.

How many balusters do I need for a 20 ft deck railing?

43, with 2 in square wood balusters and a 4 in limit. The run is 240 in, and (240 − 4) ÷ 5.5 = 42.9, so the count rounds up to 43 — which leaves a gap of 3.99 in. Under a 110 mm limit the same run takes 41, under 100 mm it takes 44, and with half-inch metal balusters it takes 53. The count is the same arithmetic every time; what changes is the limit and the width.

What is the 4 inch sphere rule?

It is the usual North American test for a guard or a stair railing: a sphere 4 in (101.6 mm) in diameter must not be able to pass through any opening, which means the clear gap between two balusters must be under 4 in. It is a safety rule about a child's head rather than a design preference, and inspection is literally a 4 in ball on a stick. Chinese practice usually states the same idea as a clear gap of no more than 110 mm, which is 4.33 in and therefore slightly looser.

Should I set the limit to 4 inches or 110 mm?

Whichever your code, your drawing or your inspector names. The two are close but not the same, and on a 12 ft run the difference is three balusters. If you are building in North America the answer is 4 in; if you are working from a Chinese drawing it is usually 110 mm, with 100 mm on some jobs — set the field to 110 mm or 100 mm and the page converts it for you. When the answer is not obvious, take the smaller limit: it costs a baluster or two and it satisfies both.

Why is the gap 3.99 in rather than 4 in?

Because the count is a whole number. The page finds the fewest balusters that keep every gap at or under the limit, and then the leftover space is shared equally — which almost never comes out at exactly the limit. A gap of 3.99 in means the run happened to divide nicely; a gap of 3.4 in means it did not, and the extra slack is harmless. What you cannot do is space them at exactly 4 in from one end, because the last gap would then be whatever is left over and could exceed the limit.

Do I measure the run including the end posts?

No — the field wants the clear run between the two end posts, because that is the space the balusters have to fill. A 20 ft railing built from two 4x4 posts has a run of about 19.4 ft (20 ft less the thickness of both posts), and using 20 ft instead buys one baluster the shorter run never needed: the extra one still fits under the limit, it is simply wasted. If the end posts are the same width as the balusters, measure from the inside face of one to the inside face of the other.

Does the page work for a stair railing?

Yes, with one adjustment: use the length measured along the rake of the rail, not the horizontal distance the stairs cover, because the balusters stand on the treads and their spacing is measured along the rail. The gap itself is still measured horizontally from baluster to baluster, which is what a 4 in sphere or a 110 mm limit is checking, so the same limit applies unchanged. Stairs also need the guard height and the load checked, which this page does not do.

• 1910.29 — Fall protection systems criteria and practices: the 4 inch sphere rule for openings in guardrails — Occupational Safety and Health Administration, US Department of Labor
• Approximate Conversions from U.S. Customary Measures to Metric — the inches-to-millimetres figures this page compares a gap against — National Institute of Standards and Technology