How to Calculate Spindle Spacing for Stairs

The Rule the Layout Must Satisfy

Spindle spacing is not just an appearance decision. In England, Approved Document K provides statutory guidance on stairs and guarding, including openings in guarding. For relevant guarding in buildings likely to be used by children under five, it uses a test based on a 100mm sphere not passing through an opening and advises that the guarding should not be readily climbable.

Use the regulations and guidance that apply to the project's nation, building type and location. The calculation below only spaces identical vertical spindles between two fixed faces; it does not design the handrail, guarding height, fixings, loads or stair.

Builder Tools defaults to a conservative 99mm maximum clear gap so rounding does not accidentally produce a displayed 100mm result. Try the Spindle Calculator before marking the rail.

Count the End Gaps

If there are N spindles between two newels, there are N + 1 gaps:

• one gap between the first newel and first spindle;
• gaps between each pair of spindles; and
• one gap between the last spindle and second newel.

Forgetting those two end gaps is the most common layout error.

The available width can be expressed as:

Total opening = (number of spindles × spindle width) + (number of gaps × clear gap)

Because gaps equal spindles plus one:

Clear gap = (total opening − N × spindle width) ÷ (N + 1)

Choose the smallest whole number of spindles that keeps the result below the permitted maximum.

Worked Example: 1,000mm Between Newels

Assume:

• clear opening between newel faces: 1,000mm;
• spindle width at the rail: 41mm; and
• chosen maximum clear gap: 99mm.

The minimum count formula is:

N = round up [(total opening − maximum gap) ÷ (spindle width + maximum gap)]

Substitute the measurements:

(1,000 − 99) ÷ (41 + 99) 901 ÷ 140 = 6.44 Round up to 7 spindles

Now calculate the actual equal gap:

(1,000 − 7 × 41) ÷ (7 + 1) (1,000 − 287) ÷ 8 713 ÷ 8 = 89.1mm

Seven 41mm spindles with eight equal 89.1mm gaps fit the opening and remain below the chosen 99mm maximum.

Six spindles would leave:

(1,000 − 6 × 41) ÷ 7 = 107.7mm

That exceeds the target, so adding the seventh spindle is necessary.

Measure the Spindle at the Right Point

Turned spindles are not one width from top to bottom. Use the section that controls the smallest opening in the finished guarding, which may be the square block where it meets the base rail and handrail. Confirm the product drawing and account for any groove, bracket or fillet system.

Measure the finished clear opening between the actual fixing faces after the newels are installed and plumb. Do not calculate from an unverified drawing dimension and expect site joinery to absorb the difference.

Marking Out Equal Centres

Once the clear gap is known, the repeating centre-to-centre pitch is:

Pitch = spindle width + clear gap

In the worked example:

41 + 89.1 = 130.1mm centres

Start with the calculated clear end gap, mark the first spindle, then step the pitch along the rail. Check the final end gap before drilling or cutting. A setting-out rod or a scrap length marked with the full pattern is safer than repeatedly reading a tape measure.

For sloping stair flights, decide whether dimensions are being set along the pitch line or as horizontal clear openings. The visible and regulatory opening can be affected by the spindle angle, rail profile and adjacent tread geometry. Use the stair and balustrade manufacturer's layout method for the chosen system.

Landings, Decking and Other Guarding

The same equal-spacing maths works for a level landing or deck, but the compliance context can differ. Guarding may also need to meet requirements for:

• where guarding is required at all;
• minimum guarding height;
• horizontal loads and fixing strength;
• climbability;
• openings beneath the bottom rail; and
• junctions at stairs, landings and corners.

Approved Document K is guidance for England. Scotland, Wales and Northern Ireland publish their own building standards or approved documents, and commercial premises can bring further requirements. Confirm the correct source before fabrication.

For workplaces, also review the HSE's guidance on reducing stair risks, which points back to suitable design, dimensions, maintenance and the applicable Building Regulations guidance.

Common Spacing Mistakes

• Dividing the opening by spindle width and ignoring gaps.
• Using N gaps for N spindles instead of N + 1.
• Rounding a borderline gap up on site.
• Measuring decorative narrow parts rather than the controlling spindle width.
• Assuming both newel posts are plumb and parallel.
• Marking every position independently and accumulating tape-measure error.
• Treating a spacing check as proof that the complete balustrade is compliant.

Final Site Check

1. Confirm the applicable design and building-standard requirements.
2. Measure between finished fixing faces.
3. Measure the actual spindle profile.
4. Calculate the minimum whole spindle count.
5. Recalculate the equal clear gap.
6. Set out from a rod or repeated pitch.
7. Dry-fit and check both end gaps.
8. Verify the complete guarding before handover.

Use the Spindle Calculator for the count and an equal-spacing diagram, then check the wider stair geometry with the Staircase Calculator.