Baluster Calculator
Space balusters evenly and keep every gap under the 4 in (100 mm) limit. Get the baluster count, exact gap, layout centres, posts and rail length for a deck or stair railing.
How to Use
- Enter the total length of railing, outside to outside of the end posts, and the largest post spacing you want (6 ft is common for wood rails).
- Enter the post width (3½ in for a 4×4) and the baluster width.
- Enter the largest gap allowed: under 4 in in the US, 100 mm in the UK. The gap you get will be just under it.
- Read the number of balusters, the exact gap and the layout spacing; the drawing shows one section.
- Or set the number of sections to 1 and enter the clear distance between two posts to space a single bay.
Worked Example
A 24 ft deck railing, posts no more than 6 ft apart, 4×4 posts (3½ in), 1½ in balusters, gaps under 4 in. 24 ÷ 6 = 4 sections, so 5 posts. The clear length in each section is (288 − 5 × 3.5) ÷ 4 = 67⅝ in.
Balusters per section: (67.625 − 4) ÷ (1.5 + 4) = 11.57, rounded down to 11, plus one = 12. Gap = (67.625 − 12 × 1.5) ÷ 13 = 3.817 in, just over 3 13/16 in. In all, 4 × 12 = 48 balusters. Layout spacing, centre to centre: 1.5 + 3.817 = 5 5/16 in. Top and bottom rails: 2 × 4 × 67⅝ in = 45 ft.
The common mistake: dividing the length by the spacing. 67⅝ ÷ 5.5 = 12.3, rounded to 12, then starting with a baluster against the post leaves a 3½ in gap at one end and an uneven look. Counting gaps as balusters + 1 makes every gap the same, including those at the posts.
Show Work
Formulas
The 4-Inch Sphere
Balusters, the turned or square uprights of a railing, were once spaced for looks alone; Renaissance stone balustrades used vase-shaped balusters nearly touching. Modern codes space them for children. A toddler’s head is about 4 in wide, so US model codes tightened an older 6 in limit to a rule that a 4 in sphere must not pass through any opening in a guard. The UK uses 100 mm for the same reason.
The same rules require guards on any deck or landing more than 30 in (600 mm in the UK) above the ground, at least 36 in tall on US homes (42 in in many commercial buildings, 900 to 1,100 mm in the UK), and strong enough to resist a 200 lb push at the top.
About This Calculator
This calculator spaces balusters or spindles evenly along a railing: the number of sections and posts, the balusters in each section, the exact gap and the centre-to-centre layout, and the rail length, in inches or millimetres. It draws a section to scale with the gaps dimensioned.
Everything runs in your browser; nothing is sent anywhere. On the open side of US stairs, the code allows a 4⅜ in sphere, and the triangle between the bottom rail, tread and riser has its own 6 in rule; check your local code.
Related calculators: Deck Calculator, Stair Calculator, and Fence Calculator.
Frequently Asked Questions
How far apart should balusters be?
In the US, the International Residential Code requires that a 4 in sphere cannot pass through a guard, so the clear gap must be under 4 in (100 mm in the UK). Most builders aim for about 3½ to 3¾ in so a slightly bowed baluster does not open up a gap that fails inspection.
How do I calculate baluster spacing?
For a clear length L between posts, baluster width w and largest gap g, the number of balusters is (L − g) ÷ (w + g), rounded down, plus one. Then the equal gap is (L − n × w) ÷ (n + 1). For 72 in with 1½ in balusters: (72 − 4) ÷ 5.5 = 12.4, so 13 balusters, and (72 − 19.5) ÷ 14 = 3.75 in.
How many balusters per foot?
With 1½ in balusters at just under 4 in gaps, about 2.2 per foot, so roughly 13 in a 6 ft section. Thinner 1 in metal balusters need about 2.5 per foot and 2 in square ones about 2 per foot.
How do I lay them out?
Mark the centre-to-centre spacing, baluster width plus gap, on a story stick. Start with a gap at one post and step along; with the equal gap from this calculator the last one lands a gap from the other post. Small errors are best spread by centring the layout in the section.
How far apart can railing posts be?
Wood deck rail posts are usually 6 ft apart, at most 8 ft, so the top rail does not sag and the guard resists the 200 lb load the code requires at its top. Aluminium and composite systems state their own maximum, often 6 or 8 ft.
How do I use the Baluster Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
One section
72 in clear between posts with 1½ in balusters: 13 balusters, 3¾ in gaps.
Deck railing
24 ft run, posts at 6 ft max, 4×4 posts: 5 posts, 48 balusters at 3 13/16 in gaps.
Metal balusters
16 ft with ¾ in round balusters at a 3½ in gap target: 3 sections, 42 balusters at 3¼ in.
UK spindles
3.6 m landing, 41 mm spindles, posts 90 mm, 99 mm limit: 24 spindles at 97 mm.
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