Carpet typically comes in 12-foot widths that you unroll across a floor to install. One of the most important steps in installing carpet is cutting it to the right size. Because it is a major investment, you don't want to cut it too short. Cutting it to the proper size requires that you take precise measurements of the room and then add some to allow for a tight fit.
Measure the width of the room by placing the end of a tape measure at one edge of the floor and stretching it to the other side of the room. Once you have the width, add 4 inches to the measurement, and write it down.
Measure the length of the room in the same manner, and add 4 inches to that measurement, as well.
Unroll the carpet in an area that has plenty of space. If the room where you're installing the carpet is small, try to find a larger room where you can unroll the carpet easily. Turn the carpet so that the rigid backing is facing up.
Use the tape measure and a marker to transfer the length measurement onto the back of the carpet. Make multiple marks across the carpet so that you can follow them as you cut.
Place a sharp utility knife or carpet cutters at the first mark, and press down as you cut all the way across the remaining marks to cut the carpet.
Use the tape measure and the marker to transfer the proper width onto the back of the carpet just as you did with the length.
Cut along the marks with the utility knife or the carpet cutter.
Roll the carpet up and move it to the room where you're installing it if you have done your work elsewhere in your home. Butt one side of the carpet up against the wall and unroll the carpet over the length of the room. You will have a surplus at the remaining walls because of the extra 4 inches that you allowed for each measurement. Proceed with securing the carpet to the floor.
Place a hookbill knife, which looks like a metal hook, on top of the carpet where it meets a wall. Press down firmly to create a crease all the way along the length of the wall, which is where you will trim the excess. Repeat the process on each remaining wall.
Place a carpet wall trimmer on the floor with the blade side facing toward the wall. Slide it as far as you can against the wall and press down as you push it forward to cut along the crease line. Repeat the process on each remaining wall to trim the excess carpet. Pick up the carpet pieces and discard them.