- 1). Measure the area of all your impermeable surfaces, including driveways, rooftops, and paths. You may measure area by multiplying the length by the width of the surface. Use meters for your measurement. Your result appears in square meters. Add all of the areas together.
- 2). Determine the average rainfall for a 24-hour period in your locale. Either measure this value using a rain gauge or ask the local environmental authority about their average rainfall statistics. Use millimeters for your measurement unit.
- 3). Multiply your average rainfall measurement by the amount of impermeable surface you have, using the same unit. You may get your average rainfall measurement in meters by dividing it by 1000. For example, if you have a house with 100 square meters of impermeable surface and 25 mm of average daily rainfall in your locale, you must multiply 100 meters by 0.025 meters, giving you a result of 2.5 cubic meters of water in runoff.
- 4). Multiply your runoff result by 1000 to get the amount of liters of runoff you get on-average daily.
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