Baluster Spacing Calculator (Balusters, Pickets and Slats)
How many balusters a stair or balcony guard needs so no gap exceeds the limit, and the exact gap and centre spacing that results, so they look evenly spaced.
Results
The dashed line marks the value you entered.
What this tool does
Guards are usually required to stop a child passing or getting a head stuck, and the common rule is that a 100 mm (about 4 in) sphere must not pass between balusters; local building rules decide the exact figure and also set the height and load of the guard, so check them. The fewest balusters that keep every gap at or under the limit is ⌈(opening − gap) ÷ (width + gap)⌉, and the gap is then shared out equally, including the two gaps against the posts. The same arithmetic spaces fence pickets, shelf slats or screen battens evenly — set the limit to whatever gap you want not to exceed.
Formula
balusters n = ⌈(opening − max gap) ÷ (width + max gap)⌉ · gap = (opening − n × width) ÷ (n + 1)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
lo | Clear opening between the posts | mm |
w | Width of one baluster | mm |
gm | Largest gap allowed | mm |
N | Balusters needed | — |
GP | Gap between balusters (and to the posts) | mm |
CC | Centre-to-centre spacing | mm |
FC | First baluster: centre from the post | mm |
Worked example
- Clear opening between the posts2400 mm
- Width of one baluster40 mm
- Largest gap allowed100 mm
- Balusters needed17
- Gap between balusters (and to the posts)95.6 mm
- Centre-to-centre spacing135.6 mm
- First baluster: centre from the post115.6 mm
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The calculation runs at full precision and only the display is rounded. If you copy an intermediate value and retype it, small differences can appear.