Decks, Fences & Siding
Baluster Spacing Calculator
Measure the clear space between two railing posts and the width of one baluster. The tool finds the fewest balusters that keep every gap under your limit and spaces them evenly.
Worked example
- Balusters = (span - max gap) / (baluster + max gap) = (72 - 3.875) / (1.5 + 3.875) = 12.67, round up to 13.
- Gap = (72 - 13 x 1.5) / 14 = 3.75 in (3 3/4 in), under the 4 in sphere rule.
Common sizes at a glance
| Size | Balusters | Gap between each | On-center spacing |
|---|---|---|---|
| 48 in, 1.5 in balusters | 9 | 3 7/16 in | 4 15/16 in |
| 48 in, 0.75 in metal | 10 | 3 11/16 in | 4 7/16 in |
| 60 in, 1.5 in balusters | 11 | 3 5/8 in | 5 1/8 in |
| 60 in, 0.75 in metal | 13 | 3 19/32 in | 4 11/32 in |
| 72 in, 1.5 in balusters | 13 | 3 3/4 in | 5 1/4 in |
| 72 in, 0.75 in metal | 15 | 3 13/16 in | 4 9/16 in |
| 84 in, 1.5 in balusters | 15 | 3 27/32 in | 5 11/32 in |
| 84 in, 0.75 in metal | 18 | 3 23/32 in | 4 15/32 in |
| 96 in, 1.5 in balusters | 18 | 3 5/8 in | 5 1/8 in |
| 96 in, 0.75 in metal | 20 | 3 27/32 in | 4 19/32 in |
Measure between posts, one section at a time
Hook the tape on the inside face of one post and read to the inside face of the next. Do not measure center to center, because the post width is not open space. Posts are rarely set perfectly, so measure each section and run the numbers for each one.
If a long rail has several sections of the same size, you can do one and multiply. Write the result on the rail in pencil so the layout matches what you measured.
The 4 inch sphere rule
The common U.S. residential code says a 4 in sphere must not pass through any opening in a guard. Most people set the maximum gap a little below 4 in, such as 3 7/8 in, so a slightly bowed baluster does not fail an inspection. The local code and a licensed professional have the final word on guards.
The tool always rounds the baluster count up. In the 72 in example that gives 13 balusters and a gap of 3 3/4 in on both ends and between each pair.
Marking the layout
The on-center spacing is the distance from the middle of one baluster to the middle of the next. Make a spacer block cut to the gap width, or mark centers along the rail with the on-center number. Start from the middle of the section if the posts are out of plumb; small errors then split evenly to both ends.
- Cut a scrap to the exact gap and use it as a spacer.
- Check every fifth baluster for plumb with a level.
- Buy one or two extra for splits and knots.
Wood versus metal balusters
Thin metal balusters take more pieces for the same rail. A 48 in section uses 9 balusters at 1.5 in wide but 10 at 0.75 in wide, and the gap math still lands under 4 in. The common sizes table below shows both for spans from 48 to 96 in.
For the posts and the deck under them, start with the deck calculator. Stair rails follow the same gap rule; lay out the steps first with the deck stairs calculator.
Questions
How many balusters for a 6 ft section?
For a 72 in clear span with 1.5 in balusters and a 3 7/8 in limit, 13 balusters with gaps of 3 3/4 in.
Should the end gaps match the middle gaps?
Yes. The tool counts one more gap than balusters, so the space next to each post equals the space between balusters.
Does this work for stair railings?
Use the level distance between posts along the stair, but the gaps on a slope look different. Check the openings with a 4 in object after installing.
How this works: formulas and typical values are listed on our method page; every data source and its license is on sources.
Last reviewed November 2026.