How to calculate stair runner length, step by step

The formula in full

Stair runner length is built up step by step rather than measured as a single continuous run, because a runner does not simply lie flat down a flight — it steps over each tread and down each riser, and wraps slightly around the nosing at the front of every tread. The formula per step is:

Tread depth + riser height + nosing wrap allowance = length used by one step.

Multiply that figure by the number of risers in the flight, add any landing runs, then apply a waste allowance. Each of those inputs deserves its own explanation, because getting one wrong by even a small margin compounds across a full flight of steps.

Measuring tread depth

Tread depth is the horizontal distance from the front nosing edge of one step to the front nosing edge of the step above it — not simply the flat, walkable part of the tread. Measure it directly rather than assuming a standard figure, because tread depth varies meaningfully between houses and even, on older staircases, between steps within the same flight.

Measuring riser height

Riser height is the vertical distance from one tread surface to the next. Like tread depth, measure several risers individually on any staircase that predates modern building regulation consistency, since older flights were often built by eye rather than to a uniform rise. A small, consistent error in riser height, multiplied across a full flight of a dozen or more steps, adds up to a noticeable shortfall in ordered length.

The nosing wrap allowance

The nosing is the rounded or projecting front edge of the tread, and a properly fitted runner wraps down over it before tucking under toward the riser below, rather than simply crossing it in a flat line. This wrap uses extra material that a flat tread-plus-riser sum does not account for, and it is the single most commonly omitted figure in a hand-calculated length, usually because it is easy to overlook when the tread and riser measurements feel like they already describe the whole step. Measure the actual nosing projection and wrap depth on your own staircase, since nosing profiles vary between different tread and stair nosing designs.

Counting steps correctly

Count risers, not treads. A flight of stairs has one more riser than it has visible full treads if the top step meets a landing rather than another tread, and counting the wrong element is a common source of an off-by-one error. Standing at the bottom of the flight and counting each vertical riser face as you go, ending at the landing, is the most reliable way to get an accurate figure.

Adding landing and hallway runs

If the runner continues across a landing, a half-landing, or into a hallway before or after the flight, add that length separately, since it is not covered by the per-step formula at all. Measure landing runs directly along the path the carpet will actually take, including any run that continues past the final riser onto level flooring. Where a runner transitions from stairs onto a landing, the join detail itself can affect exactly how much extra length is sensible to order — see the stair-to-landing transition for how that junction is usually handled.

Handling a flight with a turn

A staircase that turns, whether through a half-landing or a set of winders, needs to be treated as separate sections rather than one continuous calculation. Apply the per-step formula to each straight run individually, measure the landing or winder section on its own terms, and add all the sections together before applying waste. Treating a turning staircase as one long uniform run is a common source of under-ordering, because the geometry at a turn rarely matches the straight-flight assumptions the basic formula depends on.

Worked example: a flight with a half-landing

Measuring winders individually

Winders are pie-shaped treads that turn a corner without a landing, and their depth varies along the width of the tread, so the flat per-step formula does not apply to them directly. Measure each winder along the actual line the runner will follow — usually the centre line of the intended runner width — rather than at the wide or narrow edge of the tread, and add these individual figures to your straight-flight totals. Full detail on planning a runner's path through a set of winders is in fitting a runner around winders.

Applying a waste allowance

Ask your supplier for their recommended allowance for the specific material and pattern you have chosen rather than applying a generic percentage across every project, since pattern repeat length in particular can push the sensible figure well above a simple flat rate.

Common calculation mistakes

• Omitting the nosing wrap allowance entirely, treating each step as flat tread plus riser only.
• Counting treads instead of risers, producing an off-by-one shortfall.
• Measuring one step and assuming it applies uniformly across an older, less consistent flight.
• Forgetting to add a landing or hallway run that the carpet also crosses.
• Applying a flat per-step formula to winders instead of measuring them individually.
• Skipping the waste allowance altogether, or applying too low a figure for a patterned carpet.

Double-checking your figure before ordering

Once you have a total, sanity-check it against a rough rule of thumb: a typical domestic staircase step uses somewhere in the region of 40-50 cm of runner length depending on tread depth, riser height and nosing detail, so a 13-step straight flight landing around 5.5-6.5 m before waste is a reasonable ballpark. A figure far outside that range for a standard domestic flight is worth re-checking before you order, since it usually points to a missed or double-counted input rather than an unusual staircase.

For the full measuring process, including how width and margin are decided alongside length, see how to measure for a stair runner, and for a tool that carries out this exact arithmetic and shows its working, use the stair runner length calculator.

Calculating length for a bottom step wrap

Where a runner is finished with a bullnose or waterfall wrap around the bottom step rather than stopping flush at its edge, add an extra allowance specifically for that wrap on top of the standard per-step total. The amount depends on the exact detail chosen and the bottom step's shape, so measure the actual wrap path directly around the specific step rather than estimating, since bottom step details vary more between staircases than most other parts of the calculation. See finishing a stair runner at the bottom step for the common options and how each affects the length needed.

Calculating length for a runner that continues into a hallway

Many staircases feed directly into a hallway, and if the same runner is intended to continue across that floor rather than stopping at the final riser, measure the hallway run as its own separate length using a straightforward tape measurement along the intended path, then add it to the stair total before applying an overall waste allowance. Treat any rug pads, thresholds or doorway transitions along that hallway run as their own small allowance too, since a runner that has to clear a threshold strip or door swing sometimes needs a slightly different finishing detail than a plain hallway floor. See hallway carpet runners for how a combined stair-and-hallway project is usually planned.

How width affects the overall quantity, not just the length

Length calculations tell you how much runner to order along the direction of travel, but the total area and therefore the total quantity and cost also depend on the width chosen, particularly if you are comparing quotes based on area rather than running length. A wider runner covering more of each tread will naturally cost more per metre of length than a narrower one, and width also interacts with margin either side of the runner, which is usually decided separately from the length calculation covered in this guide. See choosing a stair runner width and stair runner margin for how those decisions are usually made alongside the length figure you calculate here.

When to get a professional measurement

A confident, careful hand measurement is entirely adequate for many straight staircases. Get a professional measurement instead where the staircase has multiple turns, a mix of winders and landings, an unusual nosing profile, or where the carpet chosen has a large pattern repeat that must be centred and matched precisely, since the cost of a measuring visit is small compared with the cost of an incorrectly cut length of patterned carpet.