Do I count treads or risers?

Treads — the surfaces you step on. A flight always has one more riser than tread, because the top floor acts as the final surface, so counting risers overstates the carpet by one step.

Why is the waste allowance higher for stairs?

Because every step is cut individually, a runner has to be centred on the stair, and any pattern must line up from step to step. Fifteen percent is a sensible floor, more with a pattern or winders.

Stair Carpet Calculator

Work out carpet for a straight flight from the step count, tread, riser and stair width.

Draft — awaiting review. Not indexed.

Carpet to order— yd²

Length of carpet: — ft
Per step: — in
Area: — ft²
Including waste: — ft²

Formula: Per step = tread + riser + nosing wrap. Length = steps × per step ÷ 12. Area = length × stair width, plus waste.

What this calculates

Stairs need far more carpet than their footprint suggests, and this works out how much. A staircase occupies only a few square feet of floor, but the carpet has to run up every riser, across every tread and around every nosing — so a flight that takes up thirty square feet of hallway can easily need seventy square feet of carpet. Estimating stairs as an area, the way you would a room, undershoots badly, which is why this is a separate page rather than an option on the carpet calculator.

How it works

Each step consumes the tread depth plus the riser height plus a wrap over the nosing — the projecting front edge — which together give the inches of carpet per step. Multiplying by the number of steps and dividing by twelve gives the total length in feet. Multiplying that by the stair width in feet gives the area, and the waste allowance is applied last. The default waste of 15% is higher than a room’s 10% on purpose: stairs are cut individually, a runner has to be centred, and any pattern must align from step to step.

Worked example

A flight of 13 steps with 10.5 inch treads, 7.5 inch risers, a 1.5 inch nosing wrap and a 36 inch width. Per step: 10.5 + 7.5 + 1.5 = 19.5 inches. Total length: 13 × 19.5 = 253.5 inches, which is 21.13 feet. Area: 21.13 × 3 = 63.4 square feet. With 15% waste that is 72.9 square feet, or 8.1 square yards. Compare that with the stair’s actual floor footprint of roughly 11 feet by 3 feet — 33 square feet — and the gap is the whole point of the calculation.

Common mistakes

Counting risers instead of treads. A flight has one more riser than tread, so counting the wrong one adds a step’s worth of carpet, or leaves you short by one. Second, forgetting the nosing wrap, which is small per step and adds up over a flight. Third, ignoring winders — turned or pie-shaped steps at a corner are wider at one end and are effectively cut individually, so a staircase with winders needs noticeably more than this straight-flight calculation gives. Measure and add those separately.

Common questions

Do I count treads or risers?: Treads — the surfaces you step on. A flight always has one more riser than tread, because the top floor acts as the final surface, so counting risers overstates the carpet by one step.
Why is the waste allowance higher for stairs?: Because every step is cut individually, a runner has to be centred on the stair, and any pattern must line up from step to step. Fifteen percent is a sensible floor, more with a pattern or winders.