Stair Carpet Calculator
Work out how much carpet a staircase needs. Enter the number of steps, tread depth, riser height, and nosing allowance in inches, plus stair width and a waste factor — you get square feet, square yards (how carpet is sold), and the running length of the strip.
Example: with Number of steps 13 · Tread depth (in) 10 · Riser height (in) 7.75 · Nosing allowance per step (in) 1 · Stair width (in) 36 → Carpet needed (with waste): 67.0 sq ft.
- In square yards7.45 sq yd
- Running length20.31 ft (20 ft 4 in) before waste
- Piece per step36 in wide × 18.75 in per step
Computed by the calculator below using its default values. Change any input to see your own numbers.
Per step you need tread + riser + about 1 in to wrap the nosing. IRC-typical steps (10 in tread, 7.75 in riser) come to roughly 19 in of carpet each.
How to measure stairs for carpet
Each step consumes carpet in two directions: across the tread you stand on, and down the riser to the next step. Add about an inch so the carpet can wrap the nosing — the rounded front edge — without going tight. A code-typical step (10 in tread, 7.75 in riser) therefore needs 18–19 in of carpet length. Multiply by the number of steps for the running length, then by the stair width for area.
Measure width at the widest step, and measure one step with a flexible tape pressed into the corner where riser meets tread — carpet follows that profile, not the straight-line distance. If steps are open on one side or wrap a post, measure those individually.
Why the waste factor matters on stairs
Stairs are cut as a series of small pieces, and every cut has to land on-pattern and on-grain, so offcuts are rarely reusable. Ten percent covers straight flights with plain carpet; patterned carpet, winders (pie-shaped steps), or Hollywood-style wrapped ends justify 15–20%. Landings are not steps — measure each landing as its own rectangle and add it to the room total.
How it’s calculated
Carpet length per step = tread depth + riser height + nosing allowance, in inches. Running length = steps × per-step length ÷ 12 (feet). Area = running length × (stair width ÷ 12) square feet; the waste factor multiplies area by (1 + waste ÷ 100). Square yards = square feet ÷ 9. Defaults reflect IRC-typical geometry: 10 in tread, 7.75 in max riser, 36 in width.
Straight box stairs only — landings, winders, and wrapped open ends need to be measured as separate pieces, and patterned carpet can require one extra pattern repeat per step.
Typical US stair dimensions
| Dimension | Typical value | Where it comes from |
|---|---|---|
| Tread depth | 10–11 in | IRC minimum 10 in |
| Riser height | 7–7.75 in | IRC maximum 7.75 in |
| Nosing projection | 0.75–1.25 in | IRC allowed range |
| Stair width | 36 in or more | IRC minimum clear width |
| Steps per flight | 12–14 | Typical 8–9 ft floor-to-floor |
Stair geometry limits: 2021 International Residential Code, section R311.7; step counts computed from typical floor-to-floor heights.
Common mistakes
- Measuring only the tread and forgetting the riser — each step needs both, plus about an inch to wrap the nosing.
- Ignoring pattern repeat: patterned carpet may need one extra repeat per step so the motif lines up, which the base math does not include.
- Treating a landing as one more step — landings are separate rectangles and usually need a seam.
- Skipping the waste factor on winders, whose pie-shaped pieces must be cut from full-width stock.
Frequently asked questions
How do I measure stairs for carpet?
Measure one step from the back of the tread, over the nosing, down to the base of the riser — typically tread + riser + 1 in, about 18–19 in. Multiply by the number of steps for length, then by stair width for area: steps × (tread + riser + nosing) × width ÷ 144 gives square feet.
How much carpet do I need for 13 stairs?
With typical 10 in treads, 7.75 in risers, a 1 in nosing allowance, and 36 in width, 13 steps need about 61 sq ft of carpet — call it 67 sq ft (7.4 sq yd) once you add 10% waste.
Why do carpet stores quote square yards?
Broadloom carpet is milled in 12-ft-wide rolls and priced per square yard; one square yard is 9 square feet. Divide your square footage by 9 to compare quotes.
What is the difference between waterfall and cap-and-band?
Waterfall installation drapes the carpet straight over the nosing down to the next tread and uses slightly less material. Cap-and-band (French cap) wraps the carpet tightly around the nosing edge for a tailored look — keep at least the 1 in allowance per step for it.
Should I still get an installer to measure?
Yes. Use this estimate to budget and sanity-check quotes, but have the installer confirm the final cut sheet before ordering — they will account for pattern match, seam placement, and any winders or landings.