The carpet formula
Carpet is priced by the square yard, so the calculation converts the room's square footage into square yards, adds waste, and multiplies by the price.
sq ft = length × width
sq yd = sq ft ÷ 9
sq yd needed = sq yd × ( 1 + waste )
cost = sq yd needed × price
There are 9 square feet in a square yard (3 ft × 3 ft), so divide square feet by 9. The waste factor covers trimming, seam matching and the fixed roll width of carpet.
Worked example
A 15 ft × 12 ft room, carpet at $28/sq yd, 10% waste:
Area: 15 × 12 = 180 sq ft.
Square yards: 180 ÷ 9 = 20 sq yd.
With waste: 20 × 1.10 = 22 sq yd.
Cost: 22 × $28 = $616.00.
Note: this estimate covers the carpet only. Padding, tack strips and installation labor are usually quoted separately, so ask whether a price is material-only or fully installed.
Why roll width matters
Carpet ships in fixed roll widths — most often 12 feet. If a room is wider than the roll, you need a seam, and aligning that seam can force extra material. A simple rectangle close to the roll width wastes little; an L-shaped room, hallways or stairs can push the waste allowance to 15-20%. When in doubt, have the installer measure before ordering.