Why centres, not gaps, is the number to work from
Gaps are what code cares about, but centres are what you actually mark out with a tape and a pencil. Centre-to-centre distance is the gap plus one spindle width, and once you have it, marking out is a repeated addition rather than a series of separate gap measurements that can compound small errors along the run.
Working out the centres
- Measure the clear run to be filled, end to end (newel face to newel face, or string end to string end for a continuous rail).
- Decide the number of spindles needed using a baluster calculator or your maximum gap limit (commonly around 100mm/4in clear gap).
- Divide the run by (number of spindles + 1) to get the centre-to-centre spacing.
- Check the resulting clear gap (centres minus spindle width) is still under your code limit.
- Mark the first and last centre in from each end, then step off the remaining centres with a story stick.
| Run | Spindles | Centres (run ÷ spindles+1) | Spindle width | Resulting clear gap |
|---|---|---|---|---|
| 36in / 91cm | 8 | 4in / 10.1cm | 1.25in / 32mm | 2.75in / 70mm |
| 42in / 107cm | 9 | 4.2in / 10.7cm | 1.25in / 32mm | 2.95in / 75mm |
| 48in / 122cm | 10 | 4.36in / 11.1cm | 1.5in / 38mm | 2.86in / 73mm |
Note that closer centres than the code minimum are common on staircases for visual reasons — many designs run tighter spacing than strictly required because it reads as sturdier and more traditional. The calculator only gives you the maximum spacing allowed; you can always choose to go tighter.
Use a story stick, not a running tape
Marking every centre from a single end of a long tape lets small reading errors accumulate. Cut a story stick with each centre marked once, then transfer marks from that stick to every tread or string position.
Spacing on winders and continuous rails
On a straight run, the calculation above is enough. On winders and continuous curved rails, the run length changes between the inner and outer edge, so the spacing calculation should be done along the pitch line — the notional line the handrail follows — rather than along the floor plan, and it is worth doing the layout in short segments (tread by tread) rather than treating the whole curve as one run.
Spacing for glass and cable infill
Glass panel balustrades and horizontal cable infill use different spacing logic. Glass panels are usually sized to fit between newels with minimal joinery, so 'spacing' is really about panel width and support post positions rather than repeated spindle centres. Horizontal cable runs must be spaced vertically tight enough (commonly under about 100mm/4in) to meet the same sphere-rule intent, and cable systems can sag over time, which tightens the practical gap needed at installation to allow for that.
Common spacing mistakes
- Marking gaps individually along the run instead of using consistent centres, which lets errors accumulate.
- Forgetting that a continuous rail over a landing needs centres recalculated for that segment's own run length.
- Ignoring spindle width when converting between gap and centre distance.
- Not accounting for cable sag, leaving cable infill too widely spaced once tensioned and settled.
- Measuring the plan-view run on a curved stair instead of the pitch-line length.
Know your numbers in 30 seconds
Enter your step count, tread depth and riser height — the stair runner calculator gives you the exact length to order, in metres and feet, with waste included.
Open the stair runner calculatorChecking your layout before fixing anything
Once you have a centre distance, dry-fit spindles along one tread or one bay of the rail before drilling or housing anything permanently. Sight along the flight from the bottom of the stairs — spacing errors of a few millimetres per spindle are invisible up close but very obvious viewed end-on along a run of ten or more.
Whatever you decide, get the numbers first: the stair runner calculator doubles as a stair runner length calculator, adding nosing wrap, landings, winders, pattern repeat and waste so you order the right length the first time.
Frequently asked questions
How do I calculate spindle spacing on stairs?
Divide the clear run by (number of spindles + 1) to get the centre-to-centre distance, then check the resulting clear gap between spindles is under your local code limit, typically around 100mm (4in).
What is the difference between spindle gap and spindle centres?
The gap is the clear space between two spindles; the centre distance is the gap plus one spindle's width. Centres are usually easier to mark out accurately with a story stick.
Can spindle spacing be tighter than the code minimum?
Yes. Codes generally set a maximum gap, not a minimum, so tighter, more closely spaced spindles are always allowed and are common for a sturdier or more traditional look.
How do I space spindles evenly on a curved staircase?
Measure and calculate along the pitch line of the handrail rather than the plan-view curve, ideally working it out tread by tread on winders.
What tool should I use to mark out spindle centres?
A story stick with each centre marked once, transferred to every tread or string position, avoids the accumulated error you get from re-measuring with a tape at each spindle.
Does cable balustrade need tighter spacing than spindles?
Often yes in practice, because horizontal cables can sag over time, which effectively tightens the gap. Many installers plan cable spacing with some allowance below the code maximum to account for this.
Do all treads need the same number of spindles?
On a straight flight, generally yes if the tread widths are consistent. Winders and landings usually need their own spacing calculation because the run length differs.
What spindle width is typical for timber staircases?
Roughly 32-45mm (1.25-1.75in) is common for timber spindles, with metal balusters usually narrower — always confirm against the specific product you are ordering.
Know your numbers in 30 seconds
Enter your step count, tread depth and riser height — the stair runner calculator gives you the exact length to order, in metres and feet, with waste included.
Work out even spindle centresEvery guide is written from hands-on measuring and fitting experience, checked against the calculator’s own maths, and re-reviewed whenever prices or methods change. Read our editorial standards.