Square yards needed including stairs and waste.
Carpet is sold in square yards. Standard carpet rolls are 12 ft wide — for rooms wider than 12 ft, you will need seams. Rooms that are 12 ft or less wide can usually be cut from a single roll width.
Don't forget to budget for carpet padding (typically $0.50–$1.50/sq ft extra) and installation labor. Higher-quality padding extends carpet life significantly and makes it feel more luxurious underfoot.