Carpet in the US is priced by the square yard, which is nine square feet. The bigger issue is roll width: almost all broadloom comes twelve feet wide, and that single constraint decides how much you actually buy.
Why a 13 foot room is expensive
A room up to twelve feet wide takes one strip with no seam. Thirteen feet takes a second strip, and the leftover from that second strip is a one-foot ribbon that is good for nothing.
This is why carpet estimates often come back well above the room's square yardage. It is not padding the quote — it is the roll width.
Seams and direction
All pieces must run the same direction. Carpet pile has a nap, and a strip laid opposite reads as a different colour under any light.
Place seams away from the main traffic path and running towards the primary light source rather than across it. A seam across a doorway will show and will wear open.
Padding is a separate purchase
Padding is bought by the same square yardage but sold separately, and quality matters more than people expect — good padding roughly doubles the life of the carpet above it.
Most residential carpet wants 7/16 inch, 6 to 8 pound density. Thicker is not better; too soft a pad flexes the carpet backing and wears it out faster.
Why carpet is sold in square yards
Carpet is one of the last materials in an American house still quoted by the square yard, and the conversion is the source of a lot of confusion at the store.
A square yard is nine square feet. A room measuring 12 by 15 feet is 180 square feet, which is 20 square yards — and the two numbers look so different that it is easy to think you are being quoted for the wrong room.
Ask which unit a price is in before comparing two quotes. A price per square foot and a price per square yard differ by a factor of nine, which is large enough to make the wrong carpet look like a bargain.
What installation adds
Carpet is not a material you buy and lay yourself in most cases. It needs stretching with a power stretcher to lie flat and stay flat, and tack strip around the perimeter.
The price you are quoted often bundles the carpet, the pad, the tack strip, the installation and the removal of the old floor. Two quotes that look far apart are frequently quoting different bundles.
Ask specifically whether furniture moving and old carpet disposal are included. Those two line items are where quotes diverge most, and neither is visible in the price per yard.