How this instrument works
Carpeting a staircase isn't the same arithmetic as carpeting a flat floor, because the carpet doesn't lie flat — it wraps over the front edge of every tread and down the face of every riser as it climbs. The standard trade method for estimating this treats each step as consuming one tread-depth plus one riser-height of material, then adds those up across every step to get the total "unrolled" length the carpet would measure if you laid the whole staircase's covering out flat on the ground.
Multiplying that total run by the stair's width converts the linear unrolled length into an area, and dividing by 9 converts square feet into square yards — the unit carpet is typically sold and quoted in. A waste allowance on top accounts for pattern matching, seams, pad trimming, and the extra material padding and re-stretching commonly consume on a staircase compared with a flat room; 10% is a commonly cited starting point, with some installers recommending more for winding or unusually narrow stairs.
This method is a standard estimating shortcut, not a substitute for a professional measurement of your actual staircase. It assumes uniform tread depth and riser height across every step and a constant stair width — real staircases with winders, landings, or tapered treads need those sections measured and estimated separately.
- Enter the number of steps in your staircase.
- Enter tread depth and riser height, both in inches — measure an actual step rather than assuming a standard figure.
- Enter stair width in feet.
- Set a waste allowance — 10% is a common starting point; consider more for winding stairs or pattern-matched carpet.
- Read Carpet needed in square yards, the unit most carpet is sold by.
Worked example — a 14-step straight staircase
A straight staircase has 14 steps, each with a 10-inch tread depth and a 7.5-inch riser height, and the stairs run 3 feet wide. Total run = 14 × (10 + 7.5) = 14 × 17.5 = 245 inches of unrolled carpet length. Converting to feet and multiplying by width: (245 / 12) × 3 = 61.25 square feet before waste.
Adding a 10% waste allowance and converting to square yards: (61.25 × 1.10) / 9 = 67.375 / 9 = 7.486 square yards — round up to whatever increment your carpet retailer sells, since most stock carpet is cut from broadloom rolls in fixed widths rather than sold in arbitrary fractional yardage.
Questions
How do you calculate how much carpet a staircase needs?
Multiply the number of steps by the sum of one tread depth and one riser height to get the total unrolled carpet run, then multiply by the stair's width and add a waste allowance to get area. Converting that area from square feet to square yards (dividing by 9) gives the figure most carpet is priced and sold in. This is the standard "unroll each step" method used by flooring estimators.
Why does each stair step need both a tread depth and a riser height of carpet?
Because carpet on a staircase doesn't lie flat — it wraps continuously from the horizontal tread surface, over the front nose, down the vertical riser face, and onto the next tread. That continuous wrap means each step consumes one tread-depth plus one riser-height of material along the carpet's length, not just the tread depth alone.
How much extra carpet should I order for stairs?
10% is a commonly cited starting waste allowance, covering ordinary seams and trimming. Consider adding more for winding staircases, stairs with landings, or a carpet with a pattern that needs to be matched step to step, all of which consume more material than a straight run of plain carpet.
Does this calculator handle landings or winder stairs?
No — this method assumes a straight run of identical steps with constant tread depth, riser height, and width throughout. A landing or a section of winder (pie-shaped) treads breaks that uniformity and needs to be measured and estimated as its own separate area, then added to the straight-run total from this calculator.
Is stair carpet sold by the square yard or the square foot?
Square yards is the traditional unit for broadloom carpet, a holdover from how it's manufactured and cut from the roll, though many retailers also list a per-square-foot price for convenience. This calculator reports the total in square yards so the figure lines up directly with most carpet quotes.