When it rains, often more water hits the earth than can be absorbed by the ground. The excess water, called surface runoff, drifts into sewers or drains. Surface runoff can erode sediment or pollute the water supply by picking up contaminants before reaching the drain. Engineers must estimate runoff when building drainage systems in order to minimize these problems.
Things You'll Need
 Tape measure
 Calculator

Measure the length and width of the site for which you want to calculate surface runoff. Note the length and width in feet. Calculate the area of the site in square feet by multiplying the length times the width.

Look up the average rainfall per year for your proposed site. Convert this data from inches to feet by dividing it by 12.

Multiply the area of your site by the average rainfall in feet in order to get the volume of rainfall on the site.

Calculate the runoff by multiplying the volume by the surface percentage. If the surface is hard (i.e. asphalt or concrete), the surface percentage is 100 percent. If the surface is grassy, the surface percentage is 60 percent.

Practice using an example. Suppose you are calculating runoff in a grassy garden measuring 24 inches by 32 inches where the average rainfall is 8 inches per year.
First convert inches into feet. The length of the garden is 24 inches/12 inches per foot or 2 feet; the width is 32 inches/12 inches per foot or 2.67 feet.
The area of the garden is therefore 2 feet x 2.67 feet or 5.34 square feet.
Convert average rainfall from inches to feet: 8 inches/12 inches per foot = 0.67 feet.
Multiply the average rainfall by the area of the site: 5.34 square feet x 0.67 feet = 3.578 cubic feet. Grassy areas have a runoff percentage of 60 percent; 3.015 cubic feet x 0.60 = 2.147 cubic feet.
References
 Photo Credit the uneven surface of the stone image by alri from Fotolia.com