Determine the area of any impermeable surfaces which contribute to the runoff. For example, if the rain from one side of a roof drains through a single drain, you must determine the area of that side of the roof. The most accurate area measurements will come from measurements taken during construction of the home.
Add up the areas of all impermeable surfaces which contribute to the runoff of a single spout. For example, if one side of the roof of your home has an area of 60 meters squared (m^2) and the garage roof has an area of 20 m^2, and rainwater from both surfaces drains into a single spout, then add the two surfaces together, giving 80 m^2.
Determine the amount of rain that falls over the period of time you are interested in. For example, you may want to know how much rain falls over a 24 hour period in order to determine if your stormwater system is adequate. You can determine the rainfall by using a rain gauge, or by using previously measured values from other sources.
Multiply the total impermeable surface area from Step 2 by the average rainfall amount determined in Step 3. For example, if we know that 20 mm falls over a 24 hour period, we would multiply that amount (converted to meters) by 80 m^2. The calculation is then 0.02 meters multiplied by 80 m^2, which gives 1.6 meters cubed (m^3).
If desired, convert the runoff volume calculated in the previous step to the more common unit of liters. To do this, multiply the answer (in meters cubed) by 1000. Therefore, in our example, the runoff volume is 1600 liters.