Fire Hydrant Flow Rate Calculator
Rounded / Smooth (0.90) – Most efficient
Square / Sharp (0.80) – Standard
Protruding (0.70) – Least efficient
Select the shape of the internal transition to the outlet.
Common sizes are 2.5 inches or 4.5 inches.
The velocity pressure measured at the stream center.
Flow Test Results
Total Flow Rate:
0 GPM
How to Calculate Hydrant Flow Rate
Calculating the flow rate of a fire hydrant is a critical procedure for municipal water authorities and fire departments. It ensures that the water distribution system can supply adequate water during fire emergencies. The flow rate is typically measured in Gallons Per Minute (GPM).
The Theoretical Discharge Formula
This calculator uses the standard hydraulic formula for discharge through a circular orifice, widely accepted for hydrant flow testing:
Q = 29.83 × c × d² × √p
- Q: Flow rate in GPM (Gallons Per Minute).
- c: Coefficient of discharge. This dimensionless number represents the friction loss caused by the shape of the hydrant outlet.
- 0.90: Smooth, rounded outlet (minimal friction).
- 0.80: Square or sharp-edged outlet (moderate friction).
- 0.70: Protruding outlet (high friction).
- d: Internal diameter of the outlet nozzle in inches.
- p: Pitot pressure reading in PSI (Pounds per Square Inch).
- 29.83: A conversion constant derived from gravity and unit conversions.
NFPA Color Coding Standards
According to the National Fire Protection Association (NFPA) Standard 291, fire hydrants are color-coded based on their rated flow capacity at 20 psi residual pressure. While this calculator determines the immediate discharge flow based on your Pitot reading, understanding these classes is essential for identifying hydrant capabilities:
- Class AA (Light Blue): 1,500 GPM or greater.
- Class A (Green): 1,000 to 1,499 GPM.
- Class B (Orange): 500 to 999 GPM.
- Class C (Red): Less than 500 GPM.
Instructions for Accurate Testing
- Inspect the Hydrant: Remove the cap from the outlet you intend to measure. Check the shape of the inner edge (rounded, square, or protruding) to determine the Coefficient (C).
- Measure Diameter: Measure the inside diameter of the outlet nozzle exactly (usually 2.5″ or 4.5″).
- Open the Hydrant: Fully open the hydrant to ensure smooth flow.
- Take Pitot Reading: Insert a Pitot gauge into the stream. The blade should be centered in the stream and held at a distance from the outlet face equivalent to half the diameter of the outlet.
- Calculate: Input your values into the form above to determine the GPM.
Note: This calculation provides the flow from a single open butt. If multiple ports are open simultaneously, the flows must be calculated separately and summed.