Ordering the wrong number of spindles is one of the most common mistakes in a staircase renovation. Buy too few and your project stalls while you wait for extras. Buy too many and you're left with a box of offcuts gathering dust. The good news is that working out the right quantity is straightforward once you understand the basic rules, and this guide will walk you through it step by step.
Why Getting the Count Right Matters
Spindles (sometimes called balusters) are the vertical bars that sit between the handrail and the baserail or tread to form your balustrade. They're not just decorative. Under UK Building Regulations Part K, the gaps between spindles must be small enough to prevent a 100mm sphere from passing through. That's roughly the size of a child's head, and the rule exists to keep young children safe.
This 100mm requirement is the single most important factor when calculating quantities. It means you can't simply space spindles wherever looks best. The maximum clear gap between any two spindles, or between a spindle and a newel post, must not exceed 99mm. Getting your count right ensures your finished staircase is both compliant and visually balanced.
How to Calculate Spindles for a Staircase Flight
The method differs slightly depending on whether your spindles sit into a baserail (a horizontal rail running along the top of the string) or are fixed directly into the treads. Let's cover both.
Spindles fixed into a baserail
When spindles slot into a baserail, they're evenly spaced in a straight run between two newel posts. Here's the process:
- Measure the distance between the two newel posts (the large structural posts at the top and bottom of your flight). Measure from face to face, not centre to centre.
- Know your spindle width. A standard softwood or oak spindle is typically 41mm square at its widest point, though turned spindles may have a narrower shaft.
- Work out the spacing. Add the spindle width to the maximum gap (99mm). For a 41mm spindle, that gives you a spacing module of 140mm (41 + 99).
- Divide the distance between the newel posts by the spacing module. Round up to the next whole number. That's your spindle count.
For example, if the distance between your newel posts is 2,800mm and you're using 41mm spindles, divide 2,800 by 140. That gives you 20. So you'd need 20 spindles, and the actual gaps would be slightly under 99mm once you distribute them evenly. Always round up rather than down to stay within regulations.
Spindles fixed into treads
When spindles are fixed directly into each tread (a common arrangement on open-plan or traditional staircases), the convention in the UK is two spindles per tread. This almost always satisfies the 100mm sphere test, because a standard tread depth (going) of around 220 to 250mm leaves just enough room for two spindles with compliant gaps between them.
To calculate quantities using this method, simply count the number of treads in your flight and multiply by two. A 13-tread staircase, for instance, needs 26 spindles. Bear in mind that the bottom tread often butts up against the newel post, so you may need only one spindle on that step. Check your specific layout before ordering.
Don't Forget the Landing Balustrade
Many people focus on the staircase flight and overlook the landing. If your landing has an open side with a balustrade, you'll need spindles there too. The calculation method is the same as the baserail approach above: measure between the newel posts, apply your spacing module, and round up.
Landing spindles are typically shorter than staircase spindles because they sit vertically rather than on the pitch (angle) of the stair. Make sure you order the correct length. At Stair Crazy, landing spindles and stair spindles are listed separately so you can pick the right size without confusion.
Common Pitfalls to Avoid
Even with the maths sorted, a few details regularly catch people out:
- Forgetting the half newels. If your staircase sits against a wall, you may have a half newel post (flat on the wall side) at the top or bottom. This affects the measurement between posts, so always measure the actual gap.
- Mixing spindle styles. Decorative turned or twisted spindles can have varying profiles. The 100mm sphere test applies at the widest gap, so if your spindle narrows significantly in the middle, the gap at that point must still be under 100mm.
- Not ordering spares. We always recommend adding two or three extra spindles to your order. Timber can occasionally arrive with a slight blemish, and having spares saves a return trip. For trade orders of larger quantities, it's standard practice to add around 5% contingency.
- Ignoring the newel-to-spindle gap. The gap between the first (or last) spindle and the adjacent newel post must also pass the 100mm sphere test. This gap is often slightly different from the spindle-to-spindle spacing, so check it during a dry fit before final fixing.
A Quick-Reference Table
To make things even simpler, here's a rough guide based on common staircase lengths and 41mm spindles in a baserail arrangement:
- 2,000mm between newels: 15 spindles
- 2,500mm between newels: 18 spindles
- 3,000mm between newels: 22 spindles
- 3,500mm between newels: 25 spindles
- 4,000mm between newels: 29 spindles
These figures include rounding up to ensure compliance. Your actual number may vary slightly depending on your spindle width and the exact distance, so always run the calculation yourself or ask a supplier to confirm.
Ready to Order?
Once you've calculated your quantities, choosing the right spindle style is the enjoyable part. Whether you're after classic turned oak, clean-lined square profiles, or contemporary metal designs, Stair Crazy stocks a wide range to suit every taste and budget. Browse our full collection of wooden spindles and metal spindles, and if you're still unsure about quantities or compatibility, our team is always happy to help you get the numbers right before you place your order.
