Baluster Spacing Calculator
How many balusters a run of railing needs so that no gap exceeds the maximum allowed — from the run length, the baluster width and the gap limit — with the actual gap, the centre-to-centre spacing, and the layout from the first post.
Building codes limit the gap between balusters so that a small child's head cannot pass — commonly 100 mm, or 4 inches.
How the baluster spacing calculator works
Building codes limit the gap between balusters so that a small child's head cannot pass — commonly 100 mm, or 4 inches. Over a run, n balusters make n + 1 gaps, so the number needed is the smallest that brings the gap under the limit; the actual gap is then a little less, and spacing them evenly means the same gap at every position. Centre spacing is the gap plus one baluster width.
Formula: n = ⌈(run − max gap) / (width + max gap)⌉; gap = (run − n × width) / (n + 1); centres = gap + width
Worked examples
| Inputs | Balusters per run | Note |
|---|---|---|
| A 2.4 m run, 32 mm balusters, 100 mm gaps | 18 | 18 balusters |
| Wider balusters | — | fewer needed |
| A short run | — | none needed |
FAQFrequently asked questions
What is the maximum gap between balusters?
Commonly 100 mm in Australia and Britain and 4 inches in the United States — the size that stops a small child's head passing through. Check the code that applies; it is an input here.
Why does the count round up?
Because one baluster fewer would leave gaps over the limit. The rounding makes the real gaps a little smaller than the maximum, which is the safety margin.
How do I mark out the spacing?
Set the first centre at a gap plus half a baluster from the post, then every centre spacing after that. Working from both ends toward the middle catches a measuring error.
Does the gap apply at the bottom rail too?
Usually — the space below the bottom rail is held to the same limit, and stairs have their own rule for the triangle at each tread.
What about glass or cable infill?
Cable railings have their own deflection rules; glass panels have none of this. This page is for vertical balusters at a fixed width.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Cement Concrete & Aggregates Australia — concrete basics and yields — bag yields, density 2,400 kg/m³
- Australian Building Codes Board — the National Construction Code
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.