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Spindle Spacing Calculator
Calculate the number of balusters and exact gap width to comply with the UK Building Regulations 100mm sphere rule. Works for landings and staircase rakes.
Total length between newel posts (inclusive)
41mm standard; 32mm square spindles
Width of each newel post at the ends
UK Building Regs require 99mm (100mm sphere rule). Adjust for other standards (default 99)
Newel posts to subtract from total balustrade length (default 2)
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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 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.
Frequently Asked Questions
Approved Document K (paras 1.39 and 3.3) requires that, in a building that may be used by children under five years of age, a 100mm sphere cannot pass through any openings in the guarding. In practice this means the clear gap between spindles must be no more than 99mm, so a child cannot be held fast or fall through. Designing all domestic guarding to this limit is standard good practice (and the approach BS 6180 takes), even though the Approved Document attaches the strict requirement to buildings that may be used by under-fives.
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.
Yes, and the limit is the same 99mm as on a landing. Approved Document K requires that a 100mm sphere cannot pass through any opening, and a sphere passes only where the narrowest clear dimension of the opening reaches 100mm. With vertical spindles the narrowest dimension is the horizontal gap, which does not change with the rake angle, so there is no allowance to open the gap up on a slope. The pitch only affects how far apart you set the spindles when measuring along the rail.
Under UK Building Regulations Part K, domestic balustrading must be at least 900mm high on stairs, landings and edges of internal floors. On external balconies and roof edges, the minimum is 1100mm. Guarding is required wherever 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.
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. For a full balustrade materials list, see our Balustrade Calculator.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.