Start measuring the length of the room where you want to install the carpeting. If there is a doorway in this section of the room, place the tape measure in the middle of the doorway, rather than against the baseboard. Write down the numbers as you take all the measurements.
Stretch the tape measure along the width of the room, measuring from the center of any existing doorway if it has one. If not, measure from one baseboard to the other.
Calculate the square yardage necessary to carpet the room by multiplying the length times the width. Write down the sum.
Repeat the calculations for each room where you are installing the same carpet. Add all the square yards together.
Add 10 percent to the total sum to allow for waste and pattern matching. Take this number with you to the carpet store.