The 4-inch sphere rule
Most UK and US building codes state that a 100mm (or 4-inch) sphere should not be able to pass through any gap between balusters. If you're calculating your spacing, always measure from the narrowest point of the balusters, not the widest part of the design. This ensures the staircase is safe for young children and prevents entrapment hazards.
| Measurement | Compliance |
|---|---|
| Sphere gap | < 100mm |
| Uniformity | Required |
| Baluster type | Must match design |
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Frequently asked questions
What is the 100mm rule?
No object larger than 100mm should fit between balusters.
How do I calculate spindle spacing?
Divide the total length by the number of gaps to ensure uniform spacing.
Does spindle design affect spacing?
Yes, you must measure from the narrowest point of the spindle shape.
Are there differences between UK and US rules?
Both follow the 4-inch (100mm) rule, though specific building code nuances can exist.
Do I need a calculator for this?
It's essential to ensure perfectly even spacing, which is very hard to do by eye.
What if I have decorative spindles?
Measure the tightest gap to ensure it meets safety standards.
How many spindles per step?
Usually 2 per step is standard, depending on the stair width and spindle design.
Is it a safety hazard if they're wider?
Yes, it is a significant risk for children and a violation of building codes.
Know your numbers in 30 seconds
Enter your step count, tread depth and riser height — the stair runner calculator gives you the exact length to order, in metres and feet, with waste included.
Plan your stair design with our stair runner calculatorEvery guide is written from hands-on measuring and fitting experience, checked against the calculator’s own maths, and re-reviewed whenever prices or methods change. Read our editorial standards.