Measure the width of the stairs (do not include molding) and record this length on a piece of paper. The runner you select cannot be wider than the staircase width; typically, the runner will be a few inches narrower than the stair width.
Position the end of the tape measure at the very bottom of the staircase, at the point where the floor meets the vertical portion of the first step.
Ask a friend or family member to hold the tape measure end in position while you measure the staircase. If a second person is not available, the tape measure's end can be taped in place.
Wrap the tape measure around each step--up the vertical face of the step, around the step edge and along the horizontal portion of the step. Continue until you reach the top of the horizontal face of the top step--this is where the runner will end. Note the measurement.
Write down the measurement on the paper mentioned in step one. Add approximately six feet to this measurement to get the required length of the runner. The extra carpet length will allow for any error in measurement, and it will provide extra runner that can be used to replace a section in the event that a portion of the runner is damaged in the future.