How do I work out baluster spacing?

Measure the clear distance between the posts, the width of one baluster, and take the maximum gap from your code book or inspector. The number of balusters is (section − max gap) ÷ (baluster + max gap), rounded up; the real gap is then (section − balusters × width) ÷ (balusters + 1). Every gap comes out equal and under your maximum.

Where do I get the maximum gap between balusters?

From the code your job is built under or from your inspector — it differs by place and by what the railing guards. This calculator never fills it in for you. Type the figure you were given and the layout keeps every gap at or under it.

Do I mark the centre or the edge of each baluster?

Either works as long as you are consistent. The tool gives the first edge mark (one gap in from the post) and the first centre mark, then the on-centre step. Hook the tape on the post face and mark cumulative distances rather than stepping one mark to the next, so small errors do not add up.

Why is my gap a strange number like 3 13/32 in?

Because it is the exact share of the space that makes every gap equal. Mark from the post with the cumulative readings the tool lists (rounded to the nearest 1/16 in or millimetre) and the tiny rounding spreads out instead of piling up at the last baluster. A spacer block ripped to the gap does the same job on site.

Baluster spacing calculator

Space between posts

Post face to post face. Inches, or feet-inches like 5' 10 1/2".

Baluster width

The face you see from the deck, measured on the actual stock.

Maximum gap

From your code book or inspector — we never fill this in.

Sections this size

sections

— balusters per section

The maximum gap is yours to supply — take it from your code book or inspector.

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Show the working — with your numbers

The arithmetic divides the space between your posts into equal gaps no larger than the maximum you type. It does not know your code, the gap under the bottom rail, stair sections on a rake, or whether the railing is strong enough — confirm those with your inspector before you build.

Type the space between the posts, the width of one baluster and the maximum gap your code or inspector gives you. The calculator rounds the baluster count up so no gap is over the limit, shares the leftover space equally, and lists the marks to make from the post as tape readings.

Worked example

Imperial. A 72 in (6 ft) section between posts, 1 1/2 in balusters, and a maximum gap of 3 1/2 in (an example figure — use the one your inspector gives you). Balusters needed: (72 − 3.5) ÷ (1.5 + 3.5) = 68.5 ÷ 5 = 13.7, round up to 14 balusters. Fourteen balusters take 21 in, leaving 51 in shared by 15 gaps: 3.4 in each, about 3 3/8 in on the tape. On-centre spacing is 1.5 + 3.4 = 4.9 in (about 4 7/8 in), so the first edge mark is 3 3/8 in from the post and the second 8 5/16 in (3.4 + 4.9 = 8.3 in).

Metric. A 1800 mm section, 38 mm balusters, an example maximum of 90 mm. (1800 − 90) ÷ (38 + 90) = 1710 ÷ 128 = 13.4, round up to 14 balusters. They take 14 × 38 = 532 mm, leaving 1268 mm across 15 gaps: 84.5 mm each, marked every 122.5 mm on centre.

On the job

Mark cumulative, not step by step. Hook the tape on the post and mark 3 3/8, 8 5/16, 13 3/16… from the list. Stepping 4 7/8 in fourteen times from the last pencil line adds each rounding and each pencil width, and the last gap ends up an eighth or two out.

Rip a spacer. Once the gap is known, rip a scrap to it and hang the balusters against the block. It is faster than marks on a long run, and a block that is a hair under the calculated gap keeps every opening under your maximum.

Count the whole job at once. Sections between posts are rarely all the same. Run each length through, add them up, and buy a few spares — a split or twisted baluster is common in a bundle.

What went wrong?

Quick referencefor balusters

Balusters: (section − max gap) ÷ (baluster + max gap), rounded up
Equal gap: (section − balusters × width) ÷ (balusters + 1)
On-centre step: baluster width + gap
First edge mark: one gap from the post face; first centre mark = gap + half a baluster
Stock sizes: “2×2” ≈ 1 1/2 in (38 mm) · 1 in ≈ 25.4 mm · 3/4 in square metal ≈ 19 mm

The formula

balusters = ⌈(L − g_max) ÷ (w + g_max)⌉ · gap = (L − n·w) ÷ (n + 1)

L is the space between the posts, w one baluster, g_max your maximum gap and n the count. A section with n balusters has n + 1 gaps, so the count is rounded up until the gaps fit under your maximum, then the space is shared equally.

Questions

How do I work out baluster spacing?: Measure the clear distance between the posts, the width of one baluster, and take the maximum gap from your code book or inspector. The number of balusters is (section − max gap) ÷ (baluster + max gap), rounded up; the real gap is then (section − balusters × width) ÷ (balusters + 1). Every gap comes out equal and under your maximum.
Where do I get the maximum gap between balusters?: From the code your job is built under or from your inspector — it differs by place and by what the railing guards. This calculator never fills it in for you. Type the figure you were given and the layout keeps every gap at or under it.
Do I mark the centre or the edge of each baluster?: Either works as long as you are consistent. The tool gives the first edge mark (one gap in from the post) and the first centre mark, then the on-centre step. Hook the tape on the post face and mark cumulative distances rather than stepping one mark to the next, so small errors do not add up.
Why is my gap a strange number like 3 13/32 in?: Because it is the exact share of the space that makes every gap equal. Mark from the post with the cumulative readings the tool lists (rounded to the nearest 1/16 in or millimetre) and the tiny rounding spreads out instead of piling up at the last baluster. A spacer block ripped to the gap does the same job on site.

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