Baluster Calculator — Railing Spindle Spacing
Find the minimum number of balusters for a railing section and the resulting gap between them. Enter the clear span between the end posts, the face width of each baluster, and the maximum allowable gap — typically 100 mm (4 in) under most building codes.
mm
mm
mm
Minimum count to keep every gap within the code limit
- 1
Span minus max gap
3,500 − 100 = 3,400 - 2
Max gap plus baluster width
100 + 45 = 145 - 3
Minimum balusters
max(0, ceil(3,400 ÷ 145)) = 24Ceiling ensures every gap is at or below the code maximum.
How does this calculator work?
Minimum balusters = ceil((span − max_gap) / (max_gap + baluster_width)); then actual_gap = (span − n × bw) / (n + 1). A 100 mm max gap is the most common building code limit. Measure the clear span between posts, not including post widths.
Formula
How this is calculated
A railing section with n balusters creates n + 1 gaps — one before the first baluster, one between each pair, and one after the last. The total span equals the combined widths of all balusters plus the widths of all gaps. To keep every gap at or below the maximum, the formula rearranges the constraint into a minimum baluster count: n ≥ (span − max_gap) / (max_gap + baluster_width), rounded up to the nearest whole number.
With that minimum count in place, the actual gap is the remaining space divided evenly: (span − n × baluster_width) / (n + 1). This is always less than or equal to the maximum gap you specified, and the balusters are evenly spaced for a uniform appearance.
The 100 mm (approximately 4 inch) maximum gap is the most common building code limit in the UK, Australia, New Zealand, and much of Europe; the US International Residential Code also specifies 4 inches for guards and balusters. Some jurisdictions use a different threshold or measure the gap diagonally on stairs — always verify with your local code before ordering materials. This calculator assumes all balusters are the same width and the span is straight and level.
Frequently asked questions
Most building codes set 100 mm (≈ 4 inches) as the maximum gap because it is too narrow for a young child's head to pass through but still allows good visibility and airflow. Always check your local code, as stair railing requirements may differ from deck or landing railing requirements.
This calculator uses the clear span — the inside measurement between the two end posts or newel posts, with no post width included. The balusters and gaps fill this clear space only. Measure from the inside face of one end post to the inside face of the other.
Use the formula actual_gap = (span − n × baluster_width) / (n + 1) to check the gap for any chosen count. If the resulting gap is below the code maximum you are free to use that count. Using more than the minimum is always code-compliant and gives a denser, more traditional look.
Also known as
TG we-Calculate Editorial Team. (2026). Baluster Calculator — Railing Spindle Spacing [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/baluster-calculator
TG we-Calculate Editorial Team. "Baluster Calculator — Railing Spindle Spacing." TG we-Calculate. 2026. https://we-calculate.com/calculator/baluster-calculator.
TG we-Calculate Editorial Team, "Baluster Calculator — Railing Spindle Spacing," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/baluster-calculator
@misc{wecalculate_baluster_calculator, title = {Baluster Calculator — Railing Spindle Spacing}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/baluster-calculator}}, year = {2026}, note = {TG we-Calculate} }
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