How to Calculate Stormwater Volume

Calculating your stormwater volume allows you to estimate your drainage requirements. Stormwater volume is the total volume of water collected on your property and is a function of the impermeable area and the amount of daily rainfall. The impermeable area might be the roof of your house, which directs water through gutters and down a common drain. You could also calculate the area of your entire property if you wished to know total volume collected. However, a portion of the stormwater volume will not be runoff, since it is absorbed by permeable areas, such as soil.

Tape measure

Instructions

- 1
  Measure the horizontal length and width of the impermeable area. This means disregarding the slope, which adds vertical distance. As an example, a sloped house would be measured from edge-to-edge at ground level, rather than from edge-to-peak-to-edge. Since rain typically falls vertically, only horizontal areas collect water.
- 2
  Multiply the length and width to calculate area. As an example, if you measured 40 feet by 30 feet, the area would be 1,200 square feet.
- 3
  Reference the average daily rainfall, which can be found through your water resources division of your county of residence. This measurement is typically expressed in inches.
- 4
  Divide the average daily rainfall, in inches, by 12 to convert to feet. As an example, 0.5 inches would be divided by 12 to achieve the average daily rainfall of 0.042 feet.
- 5
  Multiply by area by the rainfall to calculate volume. In the example 1,200 square feet times 0.042 feet results in a storm-water volume of 50.4 cubic feet of storm-water per day.
- 6
  Divide cubic feet by 7.48 to convert to gallons. In the example, 50.4 cubic feet divided by 7.48 results in a average storm-water volume of 377 gallons per day.

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