Enter the coefficient of runoff, the rainfall intensity (in/hr), and the area of drainage zone (acres) into the Drain Flow Rate Calculator. The calculator will evaluate the Drain Flow Rate. 

Drain Flow Rate Formula

The following formula is used to calculate the Drain Flow Rate. 

DFR = (C*R*A)/96.23
  • Where DFR is the Drain Flow Rate (GPM)
  • C is the coefficient of runoff 
  • R is the rainfall intensity (in/hr) 
  • A is the area of drainage zone (acres) 

To calculate a drain flow rate, multiply the coefficient of runoff, rainfall intensity, and area of drainage together, then divide by 96.23.

How to Calculate Drain Flow Rate?

The following two example problems outline the steps and information needed to calculate the Drain Flow Rate.

Example Problem #1

  1. First, determine the coefficient of runoff.
    • The coefficient of runoff is calculated to be : .56.
  2. Next, determine the rainfall intensity (in/hr).
    • The rainfall intensity (in/hr) is measured to be: 2.5.
  3. Next, determine the area of drainage zone (acres).
    • The area of drainage zone (acres) is found to be: 100.
  4. Finally, calculate the Drain Flow Rate using the formula above: 

DFR = (C*R*A)/96.23

The values provided above are inserted into the equation below and computed.

DFR = (.56*2.5*100)/96.23 = 1.45 (GPM)

Example Problem #2

The variables required for this problem are provided below:

coefficient of runoff = .75

rainfall intensity (in/hr) = 1.5

area of drainage zone (acres) = 200

Test your knowledge using the equation and check your answer with the calculator.

DFR = (C*R*A)/96.23 = (GPM)