Calculate drain flow rate with the rational method, Manning pipe sizing, or weighted runoff using rainfall, area, slope, and pipe inputs.
- All Construction Calculators
- All Rate Calculators
- Drainage Density Calculator
- Average Rainfall Intensity Calculator
- Runoff Volume Calculator
- Roof Drains Calculator
Drain Flow Rate Formula
The following formula is used to calculate the Drain Flow Rate.
\begin{aligned}
\text{DFR} &= \frac{C\cdot R\cdot A}{96.23}\quad (A\ \text{in ft}^2)\\
\text{DFR} &= 452.67\cdot C\cdot R\cdot A\quad (A\ \text{in acres})
\end{aligned}- 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 (ft² when dividing by 96.23; acres when multiplying by 452.67)
To calculate a drain flow rate, multiply the coefficient of runoff, rainfall intensity, and drainage area together. If the area is in square feet, divide by 96.23 to get GPM. If the area is in acres, multiply by 452.67 to get GPM.
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 (ft²).
- The area of drainage zone (ft²) 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 (ft²) = 200
Test your knowledge using the equation and check your answer with the calculator.
DFR = (C*R*A)/96.23 = 2.34 (GPM)
