Estimating how much concrete you'll need to add a set of stairs to the outside of your home requires some very specific calculations. You're going to need to determine how many stairs you need, as well as your ideal length, width and height for these stairs. Then, you'll need to take that information and use it to calculate how much concrete is needed for each step, and ultimately how much concrete is needed for the job overall.
Find out the length, width and depth of one of the steps you'll be making out of concrete. If your steps are leading up to a porch, for example, the length of the steps will typically be the length of the porch. Find the length in feet but the width and depth in inches. Typically the width of concrete steps is 12 inches.
Multiply your measurements. If you decided you wanted a 3 foot long by 12 inches wide by 10 inches deep step, for example, those numbers multiplied together would be 360 cubic feet of concrete per step. As one cubic yard is 27 cubic feet, divide 360 by 27 and you get 13.3 cubic yards of concrete per step.
Multiply your cubic yards of concrete per step measurement by the number of concrete steps you'll be making. This will give you the total amount of concrete you need to purchase. If each step needs 13.3 cubic feet of concrete and you want five steps, that means you need 66.66 cubic yards of concrete to complete your job.