Measure the Room
Determine how much carpet is needed by measuring the width and length of the room. Add a few inches to both measurements to allow extra carpet for thresholds and doorways. Make a rough drawing of the room and take it to the carpet retail outlet. The sales representative can advise on how much carpet will be needed and can even arrange to have the length cut.
Note: Carpet comes in widths of 12' and 15'. If the room is wider than 15', the carpet will have to be seamed. Some carpet manufacturers require that a professional carpet installer do the job if a seam is required in order to uphold the manufacturer's warranty. These regulations come under the CRI -- Carpet and Rug Institute.