Stair Carpet Calculator

How it works

The calculator adds up the carpet one step needs, then works out how the step pieces fit on the roll (wall to wall) or how long the runner must be.

Carpet per step = Tread depth + Riser height + Nosing wrap
Wall to wall:
Step pieces across the roll = ⌊ Roll width ÷ Carpet width on the stair ⌋
Rows = ⌈ Steps ÷ Step pieces across the roll ⌉
Length cut from the roll = Rows × Carpet per step × (1 + Extra ÷ 100)
Carpet to buy = Length cut from the roll × Roll width
Runner:
Length = Steps × Carpet per step × (1 + Extra ÷ 100)
Carpet to buy = Length × Runner width

⌊ ⌋ means rounding down (only whole pieces fit across the roll) and ⌈ ⌉ means rounding up (a row that is only partly used still has to be cut). The area on the stairs is Steps × Carpet per step × width and is shown for comparison.

Why stairs are measured step by step

A staircase is not a flat floor. Every step has a horizontal tread and a vertical riser, and the carpet has to cover both. Many installations also wrap the carpet around the front edge of the tread – the nose – which takes a little more on every step. So the basic unit is the length of carpet for one step: tread plus riser plus nosing wrap. Multiply that by the number of steps and you have the length of a continuous strip that would cover the whole flight.

Counting the steps

Count the treads you will carpet. Each tread is covered together with the riser below it, so a flight with 13 treads gets 13 risers in this count. At the top, the last riser leads up to the upper floor. If that riser gets carpet too, count it as one more step; the calculator then adds one tread depth more than you strictly need, which is a small spare rather than a shortfall.

Measure more than one step if the stair is old or has been altered – treads and risers are not always equal. Use the deepest tread and the tallest riser you find, so every step is covered.

Wall to wall from a roll

For wall-to-wall stair carpet, each step’s piece is as wide as the stair and as long as one step. Those pieces are cut side by side across the roll. Broadloom rolls are wide: Shaw lists products in 12 ft and 15 ft widths, and ITC lists broadloom rolls 4–5 m wide in Europe. A 36 in stair fits four pieces across a 12 ft roll, so each 17.5 in “row” cut from the roll covers four steps.

The number of rows is the steps divided by the pieces per row, rounded up. With 13 steps and four pieces per row that is 4 rows; the last row covers only one step, and the rest of it is offcut you still pay for. Length cut = 4 rows × 17.5 in = 70 in, and the carpet you buy is 70 in × 12 ft = 70 sq ft, or about 7.8 sq yd. The stairs themselves have only about 56.9 sq ft of carpet on them. That difference is normal for stairs, and it is why it helps to ask whether the offcut can be used elsewhere – for a landing, say – or whether the retailer sells a shorter cut.

Some installers cut the stair carpet as one long strip per stair width instead of separate step pieces. For the amount of carpet bought the arithmetic is the same: the strips are as wide as the stair and placed side by side across the roll.

Runners

A runner is bought at a fixed width and cut to length. There is no roll width to fill, so the length is simply the steps × carpet per step, plus any extra you add. Enter the runner width from the product page to see the area. The calculator doesn’t add anything for the bottom and top ends of the runner, binding or turn-unders – add those as extra length if your installer asks for them.

Extra length and nosing

The calculator starts with no extra and no nosing wrap, so the result is the bare geometry. Add the nosing wrap per step if your carpet wraps under the nose, and an extra length percentage for trimming. Ask your installer what they add for your stair – it depends on the carpet, the pad and the install method, so no single figure fits everyone.

What is not included

Only straight flights are counted. Winders, curved steps, open sides with exposed stringers and landings have different shapes and are cut individually; measure a landing as a small room and add it separately, and ask your installer to measure winders. Pattern matching, pad, tack strip, stair rods and fasteners are not included either.

Worked example

Common mistakes

• Counting the steps wrong. Count treads, not the steps you climb; the top floor is not a tread unless the carpet continues onto it.
• Measuring one step only. Older stairs are often uneven – measure the deepest tread and the tallest riser, or measure several and use the largest.
• Forgetting the nosing. Carpet that wraps around the front edge of each tread needs extra length per step, and on a long flight it adds up.
• Ordering by stair area alone. Wall-to-wall carpet comes off a roll of fixed width, so you pay for the full roll width over the length you cut, not just the area that ends up on the stairs.
• Treating a landing or winder as a step. Those parts have a different shape and are cut separately; this calculator covers straight steps only.

Assumptions and limits

• The flight is straight and every step has the same tread depth, riser height and width.
• Each step is covered by one piece – tread, riser and nosing wrap – as long as the step and as wide as the carpet on the stair.
• Wall to wall – step pieces are cut side by side across the roll, as many as fit in its width; the rest of the roll width is offcut. Leftover pieces are not reused elsewhere.
• Runner – one continuous strip of the runner width, as long as all the steps together.
• The extra length is added to the length cut. Pattern matching, pile direction, padding, tack strip, stair rods and fasteners are not included.

Frequently asked questions

Sources

• Solidify II 12′ and Sophistication 15′ broadloom carpet – product specifications — Shaw Floors. US broadloom carpet is sold in fixed roll widths, e.g. 12 ft and 15 ft products; the width is listed per product.
• Broadloom vs. carpet tiles: which should you choose? — ITC (carpet manufacturer). Broadloom carpet comes in rolls 4–5 m wide.
• NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.

Method reviewed November 3, 2026