Spindle Spacing Calculator

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 the newel posts from the total balustrade length (two newels by default, adjustable in Advanced Options):

Effective Length = Balustrade Length - (Number of Newel Posts × 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 — the rake does not relax it. Spindles are normally installed vertically, so the opening between two spindles is narrowest in the horizontal direction. A 100mm sphere can only pass where that narrowest dimension reaches 100mm, so the horizontal gap is the measurement that governs, regardless of the pitch angle. The pitch only changes the centre-to-centre spacing measured along the rail when you set the spindles out.

Standards

All calculations reference Approved Document K (2013 edition, for use in England) of the UK Building Regulations. Paragraphs 1.39 (stairs) and 3.3 (pedestrian guarding) require the 100mm sphere test in a building that may be used by children under five years of age. Designing all domestic guarding to a 99mm maximum gap is widely treated as standard good practice, and BS 6180 (the barrier and infill standard that Approved Document K refers to) is the reference for detailed barrier design.