# Measure rain runoff at your house

When we get one inch of rain, 0.62 gallons of water fall on every square foot of surface area.

For example, if you have a vegetable garden that is 10 ft x10 ft = 100 square feet, and you want to know how much water falls on it in a one inch rain storm, you would calculate:
0.62 gallons/square foot x 100 square feet = 62 gallons of water falling on that section.

When rain falls on natural surfaces like gardens and forests, almost all of it sinks into the ground. But when rain falls on impervious surfaces like typical roofs, parking lots, roads and driveways, the rain can’t sink into the ground and runs off into storm sewers or combined sewers.

You can calculate the amount of stormwater that runs off your home or school if you know the dimensions of impervious surfaces on the property.

1. Make a sketch of the property.
2. Measure the length and width of any buildings, driveways, sidewalks, patios and parking lots, recording them on your sketch. You may need to break odd shaped spaces into rectangles or triangles you can more easily measure and calculate.
3. Calculate the surface area of each section using the following formulas:
• rectangle: length * width