Step 1: Measure the height of the riser and the depth of the stair tread. Make sure to include the measurements of the "lip" of the stair in your figures (see photo below).
Step 2: Add the measurements of the rise and tread together and multiple by the number of stairs you have. Make sure that all your stairs have the same measurements. If not, each stair will need to be measured individually.
Step 3: We highly recommend adding approximately 10% extra to your figures for errors or cutting shortages.