Water Supply Tool
Hydrant Flow & Color Calculator
Calculate GPM from pitot readings and determine the NFPA 291 color class (Blue, Green, Orange, Red) for proper hydrant marking.
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Pitot Readings
NFPA Class
Enter pressure to see classification
Calculated Flow
0
GPM
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NFPA 291 Color Coding Standard
BLUE
Class AA
1500+ GPM
GREEN
Class A
1000 - 1499 GPM
ORANGE
Class B
500 - 999 GPM
RED
Class C
< 500 GPM
The Formula
We use the standard theoretical discharge formula:
Q = 29.83 × c × d² × √p
- Q: Flow in GPM
- c: Coefficient of discharge
- d: Outlet diameter (inches)
- p: Pitot pressure (psi)
Worked Example
Example: d=2.5", c=0.90, p=50 psi
Q ≈ 1186 GPM → Green (Class A)
Hydrant Flow Calculator: Pitot PSI to GPM
This tool estimates fire hydrant flow (GPM) using a pitot gauge reading during a flow test. It’s commonly used for water supply planning, pre-incident surveys, and hydrant marking programs. The calculation uses the standard discharge relationship: Q = 29.83 × c × d² × √p.
When to use it
- Preplans & first-due water supply checks
- Hydrant marking programs (NFPA 291 color classes)
- Training: pitot operations & estimating available flow
Important notes
- Use local SOP/SOG and water department data when available.
- Outlet geometry, hydrant condition, and residual pressure affect real-world results.
- This tool provides an estimate for operational planning.
Hydrant Flow FAQ
A common fire service formula is
Q = 29.83 × c × d² × √p where d is outlet diameter in inches, p is pitot pressure in PSI, and c is the discharge coefficient. Rounded outlets are commonly estimated with c = 0.90. Sharp/square edges may be closer to 0.80, and protruding outlets can be lower (e.g., 0.70).
Hydrants are often marked by available flow: Blue (1500+ GPM), Green (1000–1499), Orange (500–999), Red (< 500).
This page estimates flow from a single outlet using pitot pressure. If your procedure uses residual readings, multiple outlets, or department-specific adjustments, follow your local water supply testing guidelines.
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