The length formula, spelled out
The core formula fitters and suppliers use to work out stair runner length is: Length = Steps x (Tread Run + Riser Height + Nosing Wrap) + Top Allowance + Bottom Allowance, then increased by a waste percentage. Each of those terms is a single physical measurement you take off the actual staircase, not a generic assumption, because tread depth and riser height vary noticeably between a Victorian townhouse with steep, shallow steps and a modern new-build with gentler, deeper ones. Once you have the per-step figure, the maths is simple multiplication and addition, which is exactly why it lends itself so well to a calculator rather than manual arithmetic prone to transcription errors.
Tread run is measured along the flat top of the step, from where it meets the riser below to the front edge of the nosing. Riser height is measured vertically along the face between two treads. Nosing wrap is the small additional length the runner uses wrapping around the rounded front edge of the tread, typically 0.5-1.5 in (1.3-3.8 cm) depending on how pronounced the bullnose profile is. None of these three numbers are things you can safely estimate from photos or general averages - a fitter friend's staircase having 7 in risers doesn't mean yours does, so always measure your own.
Working the formula on a real staircase
Take a straight 13-step flight as the reference case. Measured tread run: 10 in. Measured riser height: 7.5 in. Nosing wrap allowance for a standard rounded nosing: 0.75 in. Per-step total: 10 + 7.5 + 0.75 = 18.25 in. Multiply by 13 steps: 237.25 in. Convert to feet: 19.77 ft. Add a combined top and bottom finishing allowance of 24 in (12 in each end) to tuck the runner cleanly at the top landing and bottom step: 261.25 in, or 21.77 ft. Finally, apply a 7% waste margin for cutting straightness and any minor pattern trimming: 261.25 x 1.07 = 279.5 in, which is 23.29 ft, so you'd order roughly 23.5-24 ft to be safe.
The metric version uses the same structure with converted units: 25 cm tread run, 19 cm riser height, 1.9 cm nosing wrap, giving 45.9 cm per step. Across 13 steps that's 596.7 cm. Add 60 cm total finishing allowance (30 cm each end) for 656.7 cm, then apply the 7% waste margin: 656.7 x 1.07 = 702.6 cm, or about 7.03 m. Note the metric and imperial results agree closely once you convert them, which is a useful sanity check when double-checking a supplier's quote given in a different unit system to the one you measured in.
Sanity-check your own numbers
If your calculated length comes out wildly different between imperial and metric versions of the same measurements, you've likely mixed up a unit conversion somewhere - always convert once at the start and work in one system throughout.
How much runner do I need?
Beyond the single worked example above, it's worth understanding how the total scales with different staircases. A shorter 11-step flight with the same 10 in/7.5 in dimensions needs roughly 18.5 ft (5.6 m) using the same formula, while a longer 16-step flight needs closer to 26.5 ft (8.1 m). Steeper staircases with taller risers, say 8 in (20 cm) rather than 7.5 in, add roughly half an inch of length per step, which across 13 steps adds another 6.5 in (16.5 cm) to the total - not huge, but enough to matter if you're ordering to the exact foot or metre and don't want to reorder.
- Measure tread run, riser height and nosing wrap on a representative step.
- Add the three figures together for your per-step total.
- Multiply the per-step total by the number of steps in the flight.
- Add a top and bottom finishing allowance, typically 12 in (30 cm) each.
- Apply a 5-10% waste margin depending on pattern and turns.
- Round the final figure up to a length your supplier sells in, usually whole feet or half metres.
Length by step count reference table
| Step count | Per-step total (US) | Estimated finished length (US) | Estimated finished length (UK) |
|---|---|---|---|
| 11 steps | 18.25 in | 18.5 ft | 5.6 m |
| 13 steps | 18.25 in | 23.3 ft | 7.0 m |
| 15 steps | 18.25 in | 25.9 ft | 7.9 m |
| 16 steps | 18.25 in | 26.5 ft | 8.1 m |
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 calculatorWhy waste margin varies
The 5-10% waste margin isn't an arbitrary padding figure - it covers real, predictable losses. Every straight cut at the top and bottom of the runner loses a small sliver to get a perfectly square edge. Any pattern with a fixed repeat, like a stripe or checkerboard, forces you to round up to the next full repeat rather than cutting exactly to the calculated length. And any winder, turn or half landing adds extra trimming as the fitter shapes the runner to follow the walked line rather than a straight run. A plain, unpatterned runner on a straight flight can often get away with the lower end of that range, around 5%, while a patterned runner with a turn should budget closer to 10-12%.
This is exactly the sequence of calculations built into a stair runner calculator - enter your tread run, riser height, step count and pattern type, and it applies the nosing wrap, finishing allowance and appropriate waste margin automatically, saving you from re-deriving the formula by hand every time you want to check a number or compare two different runner options.
Cross-checking against a supplier quote
If a supplier or fitter gives you a length that differs meaningfully from your own calculation, ask what allowances they've included. Some quotes are given as bare tread-plus-riser length with no nosing wrap or finishing allowance, expecting the installer to account for the rest on-site; others already build in a standard waste margin. Knowing your own formula-based figure means you can ask a specific question - does this length include nosing wrap and a waste margin - rather than simply trusting a number you can't verify.
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.
Still deciding on material, width or pattern? Our hub on stair runners compares weaves, widths, backings and fitting styles side by side, then hands the numbers straight to the calculator.
Frequently asked questions
What is the basic formula for stair runner length?
Length equals the number of steps multiplied by (tread run plus riser height plus nosing wrap), plus a top and bottom finishing allowance, then increased by a 5-10% waste margin depending on pattern and any turns in the staircase.
How much is a typical nosing wrap allowance?
Between 0.5 and 1.5 in (1.3-3.8 cm) per step, depending on how pronounced the rounded bullnose edge of your tread is. A gentle nosing needs less; a deep, old-fashioned bullnose profile needs closer to the higher end.
Does the formula change for a runner with a pattern repeat?
The core formula stays the same, but you round the final total up to the nearest full pattern repeat rather than cutting to the exact calculated figure, and you should use a slightly higher waste margin, closer to 10%, to cover that rounding.
How do I convert my length formula between US and UK units?
Convert your raw tread run and riser height measurements to a single unit system first - inches to centimetres or vice versa - then run the whole formula in that one system, rather than mixing units partway through, to avoid conversion errors compounding.
What finishing allowance should I add at the top and bottom?
A common allowance is 12 in (30 cm) at each end, giving 24 in (60 cm) total, which covers tucking the runner under a top landing carpet edge or bar and finishing it cleanly at the bottom step without an exposed raw edge.
Why does my calculated length differ from a supplier's quote?
Suppliers sometimes quote bare tread-plus-riser length without nosing wrap, finishing allowance or waste margin included, expecting the fitter to account for those on-site, so always ask exactly what allowances a quoted length does or doesn't include.
Do winders and landings need a separate calculation?
Yes. Measure each straight section with the standard formula, then measure the landing or winder section separately by following the actual walked line, and add all sections together with a higher waste margin, typically around 10%.
Is it worth using a calculator instead of doing the formula by hand?
Yes, mainly to avoid transcription and unit-conversion errors across multiple additions. A stair runner length calculator applies the nosing wrap, finishing allowance and waste margin consistently every time, which matters most when comparing quotes from different suppliers.
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.
Run the length formula automatically with the stair runner calculatorEvery 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.