Measure the riser of the stair by extracting the tape measure and placing it flush against the back of the riser and pressing the tape measure up to the top of the riser.
Place the measuring tape on top of the stair tread -- the flat surface of the step -- and measure the width to the end of the tread.
Add these two measurements together and then multiply that measurement by the number of stairs to get the total measurement of the carpeted stairs.