What is the 100mm sphere rule for balusters?

UK Building Regulations (Approved Document K) require that gaps in balustrading must not allow a 100mm sphere to pass through. This means the clear gap between spindles must be no more than 99mm. This rule is designed to prevent small children from getting their heads stuck or falling through the gaps.

What is the standard spindle (baluster) width?

The most common UK spindle width is 41mm (32mm square spindles are also common for modern styles). Turned spindles are typically 41mm at their widest point. When calculating spacing, always use the narrowest dimension of the spindle, as that determines the widest gap point.

Does the 100mm sphere rule apply to staircase rakes?

Yes. The 100mm sphere rule applies to all guarding, including staircase rake balustrading, landing balustrading, and gallery balustrading. On a raked balustrade, the gap is typically measured horizontally between the spindles, though building inspectors may test at any angle.

What height should a balustrade be?

Under UK Building Regulations Part K, balustrading must be at least 900mm high on stairs and 1100mm high on landings and floor edges where there is a drop of 600mm or more. These measurements are taken vertically from the pitch line (stairs) or floor level (landings) to the top of the handrail.

How do I calculate staircase dimensions for my spindles?

Use our Staircase Calculator to determine the pitch angle, riser height and going depth for your stairs. Once you know the staircase dimensions, you can use this Spindle Spacing Calculator to work out the correct number of balusters and gap width for your rake balustrade.

Spindle Spacing Calculator — Baluster Gap for UK 100mm Sphere Rule

How We Calculate This

This calculator determines the minimum number of spindles needed to comply with the UK Building Regulations 100mm sphere rule, and calculates the exact gap between each spindle.

The formula

First, we calculate the effective length by subtracting newel posts from the total balustrade length:

Effective Length = Balustrade Length - (2 × Newel Post Width)

Then we find the minimum number of spindles needed so that the gap does not exceed 99mm:

Min. Spindles = ⌈(Effective Length - 99) / (Spindle Width + 99)⌉

Finally, we calculate the exact gap:

Gap = (Effective Length - (Spindles × Spindle Width)) / (Spindles + 1)

The number of gaps is always one more than the number of spindles (a gap at each end and between each pair of spindles).

Staircase rake

For raked (angled) balustrading on staircases, the same 99mm maximum gap applies. Spindles are typically installed vertically, so the horizontal gap between them is the critical measurement. Building control officers test with a 100mm sphere at the widest point of any gap.

Standards

All calculations reference Approved Document K (2013 edition with 2019 amendments) of the UK Building Regulations. The 100mm sphere rule applies to all guarding in domestic buildings and buildings used by children under 5 years of age.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.