How to calculate runoff volume

Updated July 20, 2017

Heavy rainfall can have potentially disastrous consequences for a homeowner, resulting in damage to the home and property. Determining how much excess rainwater may result from heavy rain, over both short- and long-term time spans, is important for taking any necessary precautionary measures. Calculating the runoff volume from a single spout or drain can help you determine what type of stormwater management system you should use so you can effectively manage heavy rains in the future. With a bit of knowledge of your home, calculating runoff volume is a relatively straightforward task.

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 20m^2, and rainwater from both surfaces drains into a single spout, then add the two surfaces together, giving 80m^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 20mm falls over a 24 hour period, we would multiply that amount (converted to meters) by 80m^2. The calculation is then 0.02 meters multiplied by 80m^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 litres. To do this, multiply the answer (in meters cubed) by 1000. Therefore, in our example, the runoff volume is 1600 litres.


Do not attempt to make roof measurements alone. Instead, hire qualified professionals to make these measurements if you are unsure of the values.

Things You'll Need

  • Knowledge of home dimensions
  • Rain gauge
  • Calculator
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About the Author

Thomas Bourdin began writing professionally in 2010. He writes for various websites, where his interests include science, computers and music. He holds a Bachelor of Science degree in physics with a minor in mathematics from the University of Saskatchewan and a Master of Science in physics from Ryerson University.