Stair runner length calculator

The formula, explained

A runner follows the surface of the staircase. On each step it travels horizontally across the tread, wraps over the nosing, and drops vertically down the riser to the next tread. So the material consumed per step is:

Per step = tread depth + riser height + nosing allowance

• Tread depth — the horizontal walking surface, measured from the face of the riser to the front edge.
• Riser height — the vertical face between one tread and the next.
• Nosing allowance — extra material for the curve where the runner wraps the projecting front edge. A Hollywood installation uses slightly more here than a waterfall.

Why the numbers are never quite identical step to step

On paper, every step in a flight should be the same size. In practice, older staircases settle, treads wear unevenly, and builders working to a plan on-site often finish with a slightly taller or shorter final riser to make the flight fit the floor-to-floor height exactly. A run built to a uniform "design" figure can therefore be a few millimetres out at the top or bottom step. That is rarely a problem for the runner's overall length, but it matters for exact tread-and-riser matching if you are colour- or pattern-aligning each step, and it is one reason a fitter templates in person rather than trusting a plan alone.

Worked example

Take a straight flight of 14 steps, each with a tread depth of 26 cm and a riser height of 18 cm, and a nosing allowance of 3 cm per step.

Note that the waste figure here is illustrative, not a rule: a plain-coloured runner with no repeat might need only a small trimming margin, while a large patterned repeat could push the allowance considerably higher. Always confirm the actual figure with your supplier against the specific carpet you have chosen, as explained below.

Nosing allowance in more detail

The nosing allowance is the part of the calculation people most often get wrong, because it is small per step but compounds across a whole flight. It depends on two things: how far the tread projects past the riser face (the nosing "overhang"), and how the runner is fixed at that point.

• Waterfall fitting lets the runner fall straight down from the back of one tread to the front of the next, draping loosely over the nosing rather than being pressed into the angle. This generally uses slightly less material per step.
• Hollywood fitting tucks the runner into the joint between tread and riser at every step, following the exact profile of the nosing. This gives a crisper, more tailored line but consumes a little more material at each nosing because the carpet must wrap the curve closely rather than bridge it. See waterfall vs Hollywood for the full comparison.

If your nosing has an unusual profile — a rounded bullnose, a routed groove for a stair rod, or a metal edge strip — add a small extra margin on top of the standard allowance and confirm it with whoever is fitting the stair rods, since rod position can itself change how much wrap is needed.

What the calculator does not include

1. Landings. Measure and add these separately — see landings and turns.
2. Winders and curves. These consume more than a straight step and must be templated individually. See winder and curved stairs.
3. Bullnose or curtail bottom steps, which usually need their own piece.
4. Pattern matching waste, which depends on the repeat of your chosen carpet.

Winders: why per-step maths breaks down

The tread-plus-riser-plus-nosing formula assumes every step is a simple rectangle running straight across the stair. A winder step is wedge-shaped: it is much wider at the outer edge of the turn than at the inner edge, and the runner cannot simply be centred and cut to a uniform width without either narrowing the visible margin on one side or leaving too much uncovered tread on the other. Because of this, winders are usually measured and templated individually rather than folded into a formula, often using paper or card templates taken on site. Budget more material per winder step than for an equivalent straight step, and expect the fitter to treat this section as a separate task. The full method, including how many winders typically appear in a domestic quarter-turn or half-turn stair, is covered in winder and curved stair runners.

Pattern repeat and directional waste

Waste is not simply "a bit extra for mistakes" — it is largely driven by how the pattern on the carpet has to line up as it travels down the flight.

Ask your supplier for the repeat size in centimetres and let them calculate the waste specific to that repeat and your flight length, rather than applying a flat percentage from a different job. See choosing a pattern for how pattern choice interacts with staircase shape more broadly.

Ordering rules that avoid an expensive shortfall

• Order slightly long, never slightly short. A few centimetres of surplus can be trimmed or tucked away at a hidden edge; a shortfall usually means re-templating and re-ordering the whole flight, with the matching dye lot possibly no longer available.
• Confirm the dye lot covers the full length in one piece if the flight is long, since carpet is woven or tufted in batches and later batches can differ subtly in shade.
• Get the final quantity from whoever fits it, after they have seen the staircase, not only from your own measurement or this calculator.
• Check the return policy on cut-to-order lengths before paying — most suppliers will not accept a return once a bespoke length has been cut.

Common errors when calculating length

What to ask a fitter before you order

1. Will you template the stair in person before cutting, especially at winders?
2. What waste allowance are you using, and is it based on my actual pattern repeat?
3. Will the runner be fitted waterfall or Hollywood, and does that change the length you need?
4. Is there a spare offcut kept back for future repairs or rotation?
5. What happens if the delivered length is short by the time fitting begins?

After the length: width and specification

Length is only half the order. Set the runner width and side margins from your clear tread width, then check the width against the roll sizes your carpet comes in, or specify a cut and bound custom width. The full measuring method, including how to handle awkward flights, is in the measuring guide. Once you have both figures, it is worth reading how they interact with underlay thickness, since a thicker pad changes the effective nosing wrap slightly and can shift your length calculation by a small but real margin on longer flights.

Frequently confused terms

Length is the total run of carpet from top to bottom, including landings. Run sometimes refers only to the straight sections between turns. Drop is occasionally used for the vertical riser component alone. When getting quotes from more than one supplier, confirm which of these terms they mean before comparing figures, since a mismatch in terminology is a common source of quotes that look different but are actually describing the same job.

Rounding and the "just-in-case" metre

In the world of bespoke stair runners, there is a significant price and stress difference between being 10cm over and 10cm under. If your calculation suggests you need 7.2 metres, it is almost always safer to order 8 metres. This "just-in-case" material serves three purposes: it allows the fitter to avoid a structural flaw or stain in the middle of a roll, it provides a buffer if a step measurement was slightly off, and the remaining offcut is invaluable for future repairs or for "rotating" the runner later to move wear away from the nosings.

Spiral and flared stairs: when the formula fails

The standard "tread + riser + nosing" formula assumes the staircase travels in a single direction. Spiral staircases and flared bottom steps (where the step gets wider as it approaches the floor) defy simple arithmetic. For these, the runner must be templated using the "outer string" measurement — the longest possible path the carpet has to travel. On a flared step, the width is also changing, meaning you may need to order a wider roll of carpet than for the rest of the flight, leading to significantly more waste. If your stairs are anything other than rectangular, treat the calculator's result as a "minimum possible" figure and expect a professional fitter's estimate to be 20-30% higher. See thewinder guide for more on complex shapes.

The cost of a measurement error

If you under-order by even a few centimetres, you cannot simply "patch in" a new piece of carpet. Any join in a runner is a point of weakness and a visible blemish. Because carpet is produced in batches (dye lots), a second order of the "same" carpet may be a slightly different shade, making a repair obvious. This is why professionalmeasurement is the most critical step in the process. It is also worth considering the widthcarefully; a runner that is too wide for the stairs can't be "squeezed" in without buckling.