Step 1: Enter the Square Feet Drained Step 2: Choose the Coefficient of Runoff 0.35 (Grass) 0.4 (Sand) 0.45 (Loam) 0.5 (Gravel) 0.6 (Clay) 0.7 (Compact Gravel) 0.8 0.9 1.0 (Concrete/Asphalt/Roof) Step 3: Choose the 100 Year Rainfall 1 1.5 2 2.5 3 3.5 4 4.5 5 Note: To get your 100 Year Rainfall, click here for our Precipitation Index. Step 4: Press the Calculate button for results Step 5: View your results below Liquid Flow: Gallons per Minute (GPM) Cubic Feet of Water per Second (CFS)