Stair Spindle Spacing Calculator UK

This calculator works out the exact spacing for stair spindles (balusters) to UK building regulations, which require a maximum 100mm gap between them. You enter your stair’s tread length and the spindle width, and it returns the number of spindles needed and the precise spacing between each one.

Formula

The calculation uses two simple formulas based on the UK’s 100mm rule:

Number of spindles: n = ceil((L + W) / (100 + W))

Spacing between spindles: S = (L - (n × W)) / (n - 1)

• L = length of the stair tread (the horizontal distance the spindles cover), in millimetres
• W = width of one spindle, in millimetres
• n = number of spindles required (rounded up to a whole number)
• S = gap between each spindle, in millimetres (must be ≤ 100mm)

The first formula works out how many spindles fit while keeping every gap at or below 100mm. The second formula then divides the remaining space evenly between the spindles to give the final spacing.

Worked Example

Using the default values from the calculator (tread length 900mm, spindle width 32mm):

1. Calculate the number of spindles: (900 + 32) / (100 + 32) = 932 / 132 = 7.06. Rounded up, this gives 8 spindles.
2. Calculate the total width taken by spindles: 8 × 32 = 256mm.
3. Calculate the remaining gap space: 900 – 256 = 644mm.
4. Divide by the number of gaps: there are 7 gaps between 8 spindles, so 644 / 7 = 92.0mm.

So for a 900mm tread with 32mm spindles, you need 8 spindles spaced 92mm apart. This satisfies the 100mm maximum gap rule.

Why 100mm? The UK Building Regulations Background

The 100mm limit comes from UK Building Regulations (Approved Document K, Part K of the Building Regulations in England and Wales, with similar rules in Scotland and Northern Ireland). The rule exists to prevent children from getting their heads stuck between balusters. A 100mm sphere (roughly the size of a small child’s head) must not be able to pass through any gap in the guarding. This applies to stairs, landings, ramps, and any other guarding where there is a drop of more than 600mm.

The regulation is not just a suggestion; it is a legal requirement for new builds and most renovations. Building control officers will check spindle spacing during inspections. If you are replacing spindles on an existing staircase, the same rule applies, even if the original staircase had wider gaps.

How to Measure Your Stair Tread

Getting the tread length right is the most common source of error. The tread length is the horizontal distance along the pitch line of the stair, between the inner faces of the newel posts (or the outer edges of the stringers if there are no newels). Here is how to measure it:

1. Place a tape measure at the front edge of the tread, where it meets the riser below.
2. Measure along the tread surface to the back edge, where it meets the riser above.
3. This gives you the going (the horizontal depth of the step). For a straight flight of stairs, the tread length is the same for every step.
4. If your stairs have a winder (angled steps), measure each tread individually, as the length can vary.

If you are measuring an existing staircase, measure from the inside of one newel post to the inside of the other, along the pitch line. Do not measure along the diagonal of the stair stringer; the regulation applies to the horizontal projection.

Choosing the Right Spindle Width

The spindle width is the thickness of the baluster at its widest point. Common UK spindle widths are:

• 32mm – the most common square or turned spindle size for modern stairs.
• 41mm – a thicker, more substantial spindle often used in period or high-end properties.
• 19mm – slim metal or wrought-iron spindles, popular in contemporary designs.

If you are using turned spindles, measure the widest part of the turning, not the narrow neck. The widest point is what creates the smallest gap, so it is the critical dimension for compliance. If you are unsure, measure several spindles and use the largest measurement.

Common Mistakes and How to Avoid Them

• Measuring the diagonal instead of the horizontal: Always measure along the tread, not along the slope of the stair. The 100mm rule applies to the horizontal gap, not the distance along the angle.
• Forgetting the newel posts: The gap between the top spindle and the newel post is also a gap and must be ≤100mm. Include the newel posts in your total length calculation.
• Rounding down the number of spindles: Always round up. If the calculation gives 7.06 spindles, you need 8. Rounding down to 7 would create gaps over 100mm.
• Using the wrong spindle width: As mentioned, use the widest part of the spindle, not the average.
• Not accounting for the gap at the bottom and top of the stair: The first and last gaps are between the newel post and the first/last spindle. These are included in the calculation if you measure the full distance between newels.

Costs and Materials

The cost of spindles varies widely depending on material and style. Softwood spindles (pine, hemlock) are the cheapest, typically a few pounds each. Hardwood (oak, ash) can be two to three times more expensive. Metal spindles (steel, wrought iron) range from mid to high cost, depending on the finish.

You will also need:

• Spindles – number from the calculator, plus 5-10% for wastage and cutting errors.
• Baserail and handrail – if you are replacing the whole balustrade, you will need these too.
• Fixing adhesive or screws – check the manufacturer’s instructions for the correct type.
• Wood filler and paint/varnish – for finishing.

If you are hiring a carpenter, labour costs will be the largest expense. For a DIY job, the spindle cost is the main material cost. Always buy a few extra spindles; cutting errors and damaged pieces are common.

Safety and Building Control

Stair spindles are a safety feature. The 100mm rule is not just about aesthetics; it prevents serious injury. A child can easily get their head stuck between wider gaps, and the result can be fatal. Even if you are not required to get building control approval (for example, if you are replacing like-for-like spindles on an existing staircase), you should still follow the 100mm rule.

If you are making any structural changes to a staircase, or if your property is listed or in a conservation area, you may need building regulations approval or planning permission. Check with your local authority before starting work. If you are unsure about any aspect of the installation, consult a qualified builder or structural engineer.

Frequently Asked Questions

Does the 100mm rule apply to the gap between spindles or the gap between the spindle and the handrail?

The rule applies to all gaps in the guarding, including between spindles, between spindles and the handrail, and between spindles and the baserail or tread. Any opening that a 100mm sphere can pass through is not allowed. So when you install spindles, ensure the gap at the top and bottom of each spindle is also ≤100mm.

What if my stairs are open on both sides?

If your staircase is open on both sides, you need spindles on both sides. Each side is treated as a separate balustrade, and the 100mm rule applies to each side independently. You will need to measure each side separately, as the tread length may differ if the stair is not symmetrical.

Can I use a different spindle spacing for the landing than for the stairs?

Yes, the landing is a separate guarding situation. The same 100mm rule applies, but the length you measure is the horizontal distance along the landing edge, not the stair tread. You can use the same calculator with the landing length as the input.

What is the maximum gap between spindles if I have a very long stair tread?

The maximum gap is always 100mm, regardless of the length of the tread. For longer treads, you simply need more spindles. The calculator automatically adjusts the number of spindles to keep the gap at or below 100mm.

Do I need to space spindles evenly if I have a newel post in the middle?

If you have an intermediate newel post, you should treat each section between newels as a separate calculation. Measure the distance between the two newel posts (or newel and wall), and run the calculator for each section. This ensures the gaps are even within each section, which looks better and keeps the 100mm rule.

Can I use this calculator for glass balustrades or wire rope systems?

No. Glass panels and wire rope systems have different regulations. For glass, the panels themselves must be safety glass and the fixings must be secure; the 100mm rule applies to the gap between panels and the floor/handrail. For wire ropes, the vertical spacing between horizontal wires must be ≤100mm. These systems are covered by different parts of the building regulations, so use a specialist calculator or consult a professional.

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in l | Stair tread length | mm | 900 | 0 | 5000
in w | Spindle width | mm | 32 | 0 | 200
out n | Number of spindles | - | ceil((l + w) / (100 + w)) | 0
out s | Spacing between spindles | mm | (l - (n * w)) / (n - 1) | 1