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.

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## 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**

- First, determine the coefficient of runoff.
- The coefficient of runoff is calculated to be : .56.

- Next, determine the rainfall intensity (in/hr).
- The rainfall intensity (in/hr) is measured to be: 2.5.

- Next, determine the area of drainage zone (acres).
- The area of drainage zone (acres) is found to be: 100.

- 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)