Figuring how much carpeting is needed for a basic rectangular space is pretty straightforward: Multiply the length of the carpet by the width, in feet. Divide that by nine to determine square yardage.
Stairs are trickier. Estimate length by counting the number of steps and allowing 18 inches of carpeting per standard step (eight-inch riser, 10-inch tread). Decide how wide the carpet will be. Round off the two resulting measurements to the nearest foot and multiply. Divide that result by nine to get a ballpark figure for square yardage.
Retailers will measure the space to get a more exact figure. They will probably add about 10 percent to account for irregularities in the room and matching patterns. Cutting, binding and installation may carry additional costs. Some stores charge extra for 90-degree turns and for hauling away old carpet.
Some rug stores sell room-size carpet remnants left over from other jobs. You can often find a piece large enough to cover a set of stairs and connecting hallway. The selection will be considerably smaller, but if you're not too fussy, you can get high-quality carpeting for a low price.
Rods installed over the carpet at the base of each step are purely decorative. They provide a nice finished look, especially in formal settings, but do not actually hold the carpet in place. "If you have (a) grand stair, like the one from 'Gone With the Wind,' they are appropriate," says Baltimore designer Mona Hajj.