Equal spacing· 3 min read
Baluster Spacing: How to Space Balusters So No Gap Exceeds the Limit
How to space railing balusters evenly within a maximum gap: a worked 1.5 m section for a 120 mm, 4-inch-sphere and 80 mm limit, and where to mark each one.
A railing is not just about looks. The gap between balusters has a limit, and it applies to every gap — including the one next to the main post. So you can't just start at one post and work along with a fixed gap. You need to work out how many balusters the section requires and then split the gaps evenly.
What the rules say
Limits differ by country and building type. Two examples: in Slovakia, Decree 532/2002 (§ 28 (6)) says gaps in a railing must not be wider than 120 mm in apartment buildings and 80 mm in facilities for children. In the US, the International Residential Code (R312.1.3) says required guards must not have openings that allow a 4-inch (102 mm) sphere to pass; on the open sides of stairs the limit is a 4-3/8-inch sphere. Local codes can amend these values.
Always check which rules apply to your building — in your plans, with your local building department or with the designer. The calculator only lays out the balusters; it doesn't decide which limit applies.
Example: 1,500 mm section, 40 mm balusters
The clear width between the main posts is 1,500 mm (about 59 in) and the balusters are 40 × 40 mm. In the calculator, choose “Gap must not exceed” and at the ends “Same gap as between elements”, so the gap at the posts also respects the limit. For the 4-inch sphere rule we use a limit of 98 mm, a little under 4 in, for a safety margin.
| Gap limit | Balusters | Actual gap | Centre to centre |
|---|---|---|---|
| 120 mm | 9 | 114 mm | 154 mm |
| 98 mm (under the 4-inch sphere) | 11 | 88.3 mm | 128.3 mm |
| 80 mm | 12 | 78.5 mm | 118.5 mm |
With a 120 mm limit you need 9 balusters and a gap of 114 mm. Eight balusters would leave gaps wider than 120 mm, so the calculator adds a ninth. Under the 4-inch rule the same section needs 11 balusters with a gap of 88.3 mm (about 3-1/2 in).
Why not start at the post with a 98 mm gap
It seems logical: set the gap to exactly 98 mm and keep going. But with an exact 98 mm gap, 10 balusters fit and the rest is split between the ends — leaving 109 mm at each post. That is over the limit. For railings, always work with a maximum gap and the same gap at the ends.
Marking positions (98 mm limit)
- 1The first baluster starts 88.3 mm from the main post.
- 2Each next one starts 128.3 mm further on: 216.7, 345, 473.3 mm …
- 3The sixth baluster is in the middle of the section — its centre is at 750 mm.
- 4The eleventh baluster starts at 1,371.7 mm, leaving 88.3 mm at the other post.
What to watch out for
- Measure the gap as the clear space between balusters, not centre to centre. Centre to centre is always larger by one baluster width.
- On a stair railing the section is sloped. Enter the horizontal clear width and the horizontal baluster width — but the gap that counts is the one square to the balusters, so check it after fitting.
- If the railing is welded or made to measure, confirm baluster sizes and spacing with the maker and the plans.
- The calculator doesn't check the strength of the railing or its fixings. That is a job for the designer or structural engineer.
The limits quoted come from Slovak Decree 532/2002 (version in force from 1 March 2020) and IRC R312.1.3. Code editions and local amendments change — check the current rules for your project before you build.