Baluster Calculator
Count the balusters a railing run needs from its length, posts and spacing.
Metric and imperial lengths, with feet/inches and metres/centimetres entry.
Updated October 20, 2026
Introduction
A baluster row is a run of short vertical boards with an opening between each one, and the number you need is decided by two things: how much of the run the posts already occupy, and how much room one baluster plus one opening takes. This page does that division and rounds the answer up, so the last opening closes instead of running wide.
It answers a buying question, and only that one. Deck, porch and landing railings are the obvious cases; so are fence panels with decorative spindles and the guard around a mezzanine floor. What it will not tell you is whether your chosen opening is permitted where you live. The spacing box raises a warning at four inches, and the rule that actually applies is the one enforced in your area.
The two inputs people usually get wrong are the run and the opening. The run is measured from the outer face of the first end post to the outer face of the last, because the posts are part of the rail and their width comes back out of the arithmetic. The opening is the clear gap between two balusters, not the distance from the centre of one to the centre of the next. Get those two right, and the rest is a division you can check on paper.
6 length units
The run reads in feet and inches in two boxes, or as one figure in metres or centimetres
Whole balusters
The quotient is rounded up, never down
4 in opening flagged
A wider gap warns while the count still updates
Quick start
Measure the run outside to outside
Hook the tape on the outer face of the first end post and read at the outer face of the last one. If the posts are not up yet, measure the clear space you are aiming for and add one post width at each end, so the run you enter matches what the finished rail will span.
Count every post standing in that run
End posts included, plus any intermediate post that carries the rail over a long span. Each of them takes its own width out of the run before the balusters are counted, so a forgotten post is a count that comes out a little too high.
Enter the post width and the baluster width
Post width across the face the balusters run into. Baluster width at the widest point of the baluster — the belly of a turned spindle rather than its neck, because it is the widest part that decides the opening a body could pass through.
Choose the clear opening you want
The opening, not the pitch. Centres of 5 in with 1.5 in balusters leaves a 3.5 in opening, and it is the 3.5 in the page wants. Once your opening passes 4 in (101.6 mm) the box turns red, but it still accepts the figure and still counts, so you can see what a wider gap would save before you go back to the drawings.
Read the count
A 16 ft deck run with three 4 × 4 posts (3.5 in across), 2 × 2 balusters (1.5 in across) and a 3.5 in opening. In inches the run is 192:
The page shows 37. Read the 36.3 as the useful part: 36 openings close exactly and a third of one is left over, so the 37th baluster is the one that closes the row. Order a little more than 37 — the ordering advice below explains why.
What the formula does with your numbers
One division and one round-up. The work is in getting the two quantities on either side of that division right: the clear run left between the posts, and the room one baluster plus one opening takes up.
The opening box wants the gap, not the centres
This is the input that decides whether the answer is usable. A tape hooked on the centre of one baluster and read at the centre of the next gives the pitch. The opening is smaller, by exactly one baluster width:
So 1.5 in balusters at 5 in centres leave 3.5 in of air. Type 5 into the opening box and the page fits fewer, wider-spaced balusters than the rail should have, because it believes every gap is 5 in. Type 3.5 and the arithmetic matches what you will build.
Why the posts are taken out first
A post already fills the space a baluster would have occupied, and the run was measured across the outer faces of two posts. Every post width therefore comes out before anything is divided. It also explains a result that surprises people: add a post and the count can drop, because that post consumes run that balusters would otherwise have used.
Why it rounds up, and what the remainder means
A quotient of 36.3 is not 36.3 balusters. It means 36 openings close exactly and part of the run is still open, so a 37th baluster is needed to close it. A quotient that lands on a whole number stays there — the 20 in the second example below — and the page does not quietly add a spare.
The fraction you did not round away is the slack you distribute when you fit the row. With the 42 balusters of the first example, the balusters and the 41 openings between them use 578 cm of a 582 cm run, so there are 4 cm to share between the two end openings. That is why a row is set out from the centre outwards rather than from one end.
When the box turns red
Once the opening passes 4 in (101.6 mm) the spacing box turns red and explains itself, and the count beside it still updates. That is deliberate: a wider opening usually means fewer balusters, and seeing the number is the quickest way to judge whether the saving is worth redesigning the spacing for. The page marks a common maximum, not the rule you are held to; where you build may allow less.
Worked examples
A 6 m deck run, metric
Situation: A 6 m section of deck railing between two 9 cm posts, built from 4 cm square balusters with a 10 cm opening — just inside the four-inch mark.
Inputs
Result
In practice: Order 46 and expect to fit 42. When the row goes down, 42 balusters plus the 41 openings between them use 578 cm, leaving 4 cm to split between the ends — mark the centre and work outwards both ways.
A 3 m rail at 12 cm openings
Situation: A 3 m landing rail between two 10 cm posts, with 4 cm balusters, and an owner who would like 12 cm openings to buy fewer boards.
Inputs
Result
The gap box turns red at 12 cm: it is past the 4 in (10.16 cm) figure the page watches. Nothing is blocked, and the 18 is the count for the spacing as typed.
In practice: The version at 10 cm openings needs 280 ÷ 14 = 20 balusters, two more boards, and openings that sit inside the usual four-inch figure. Make that choice with the rule that applies to the building in front of you, not with the count alone.
Measuring and ordering advice
Measure where the baluster is widest
Turned spindles are narrow at the neck and wide at the belly, and it is the belly that decides the opening. Measure there and enter that width; the count already allows for the shape, and the wider figure keeps the openings on the safe side of the limit.
Order spares, not an exact fit
Baluster stock bows, splits and arrives with a rough end, and the last piece in a row is often trimmed to suit the frame. Ten per cent over the counted figure is the usual allowance; on a short run that is two extra boards.
If you are costing the rail stock by volume rather than by the piece, the board foot calculator turns thickness, width and length into board feet.
Set the row out from the centre
The count tells you how many, not where the first one goes. Mark the centre of the clear run, step outwards in pitches of baluster plus opening, and divide the leftover millimetres between the two end openings so both ends look the same as the rest.
Four inches is a prompt, not an approval
The warning fires because four inches is the figure the page watches, not because it knows your address. Jurisdictions differ, and a stair, a raised deck or a public building can carry its own stricter rule. Check that before the first cut.
Questions
Can I work backwards from a number of balusters I want to use?
Not on this page: the count is the only thing it works out, and a rounded-up number cannot be turned back into a single opening. You can do it by hand with the same quantities. With balusters there are openings between them, so the opening that fits a clear run exactly is:
Solve it for the count you have in mind, then compare that opening with the rule that applies where you are building before you commit to the layout.
Does it handle a stair railing?
No, and it is worth being clear about why. A stair rail rises at an angle, so the length that matters is the sloped length rather than the plan length, and the opening that is measured on a rake is not the horizontal gap this page divides by. Work a stair from its sloped dimensions, and check the triangular opening that forms at the bottom of the rake as well as the gaps between the balusters.
My end posts are wider than the posts in between. What do I enter?
Enter the wider of the two sizes if you want an upper bound on the count, because subtracting more post width always yields fewer balusters. If you need the exact figure, take the two end posts and the intermediate ones separately: subtract their combined width from the run yourself, then let the page divide what is left by the baluster plus opening.
Is the number the count to fit, or the count to buy?
To fit. It is the exact number the run needs when the openings come out as typed, with no allowance for the board that splits or the piece you cut 5 mm short. Buy above it — see the ordering advice above — and keep the off-cuts, because a short run often needs one more piece than the arithmetic suggests.
How far to trust the count
- A quantity, not a compliance check. The page flags an opening wider than 4 in (101.6 mm) and says so; it does not know the code you build under, the height of the rail, the use of the building or the size of the gap the rule allows. The authority that enforces that rule where you build has the last word.
- One straight horizontal run at a time. Raked stair rails, railings that step down a slope and curved sections are outside the model; count each straight run on its own and add the pieces up.
- Every post is assumed to be the same width. Heavier end posts are the usual exception, and they subtract more run than the page took out. Enter the wider size to stay on the safe side of the count, or do that subtraction yourself.
- The count assumes the openings you typed. A frame out of square, stock that bows and a rail that springs as it is fitted all move the gaps, so the finished row is set out to the tape rather than to the number. Treat the leftover centimetres as adjustment, not as error.
- Pieces, not a material list. Rails, newel posts, fasteners, finish and the labour to fit them are not in the answer, and the count carries no waste allowance of its own.
- A correct count does not make a safe guardrail. Where the rail protects a drop, its height, the size of the openings and the way it is fixed all matter, and a rail that flexes or pulls out of its posts is not rescued by good arithmetic. If the work is inspected, settle the design with the inspector or a qualified builder before installing it.
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