Fire Hydrant Flow Calculator
Pitot PSI → outlet GPM · Flow test → projected GPM at 20 PSI + color class
Uses the standard fire service formula Q = 29.83 × c × d² × √p.
Free, mobile-ready, no login required.
Enter Pitot Readings
Project Available Flow from Test Readings
Qtarget = Qtest × ((Pstatic − Ptarget) / (Pstatic − Presidual))0.54. Static and residual readings describe the pressure drop during the test. The color class uses Qtarget at 20 PSI, not the pitot outlet flow. Changing the target does not change the color classification.
This tool models one flowing outlet. Reference: USFA Water Supply Systems, Volume II, Chapter 3. Small measured pressure drops can make projections unreliable.
NFPA 291 Hydrant Color Classification
NFPA 291 Recommended Practice for Fire Flow Testing and Marking of Hydrants defines color-coding of hydrant bonnets and caps based on available fire flow at 20 PSI residual pressure. Engine companies use these colors for rapid water supply assessment during preplanning and on-scene operations.
BLUE
GREEN
ORANGE
RED
The Pitot Flow Formula: Q = 29.83 × c × d² × √p
This is the standard theoretical discharge formula used across IFSTA, NFPA, and NFA references for calculating fire hydrant flow from a pitot gauge reading.
| Variable | Meaning | Unit | Typical range |
|---|---|---|---|
| Q | Flow rate (result) | GPM | — |
| 29.83 | Constant (unit conversion factor) | — | Fixed |
| c | Discharge coefficient | Dimensionless | 0.70 – 0.90 |
| d | Outlet internal diameter | Inches | 2.5 – 4.5 |
| p | Pitot gauge pressure | PSI | 10 – 100 |
Worked Example
Setup: 2½" rounded outlet (c = 0.90), pitot reads 50 PSI.
Q = 29.83 × 0.90 × (2.5²) × √50 = 29.83 × 0.90 × 6.25 × 7.07
Outlet flow: ≈ 1,186 GPM. This alone does not establish a color class. With static 70 PSI and residual 50 PSI, the 20 PSI projection is approximately 1,946 GPM (Class AA).
Quick Reference: Common Pitot Readings (2½" Outlet, c=0.90)
Use this table for fast field estimates. Values are approximate — always use the calculator for your specific diameter and coefficient. Pitot-only values cannot establish hydrant color classification.
| Pitot PSI | Estimated outlet GPM |
|---|---|
| 10 PSI | 530 GPM |
| 20 PSI | 750 GPM |
| 30 PSI | 918 GPM |
| 40 PSI | 1,060 GPM |
| 50 PSI | 1,186 GPM |
| 60 PSI | 1,298 GPM |
| 70 PSI | 1,402 GPM |
| 80 PSI | 1,498 GPM |
| 90 PSI | 1,589 GPM |
| 100 PSI | 1,675 GPM |
Values calculated using Q = 29.83 × 0.90 × 6.25 × √p (2.5" outlet, rounded c=0.90). For other diameters, use the calculator above.
How to Use This Calculator
- Select your outlet diameter. Measure the actual test outlet. A 2½" connection is commonly a hose outlet; larger pumper outlets may be 4" or 4½". Use the diameter and outlet geometry from the test, not a generic port name.
- Choose the discharge coefficient. For most modern hydrants with rounded outlets, use 0.90. If you are unsure, verify outlet geometry with the water authority. Do not treat 0.90 as a conservative fallback; a larger coefficient produces a larger flow estimate.
- Open the hydrant fully and take your pitot reading. Hold the pitot tube centered in the stream at approximately one-half the outlet diameter from the end. Wait for the gauge to stabilize before recording.
- Enter the PSI and click Calculate. Pitot-only mode returns outlet GPM. Enable flow-test mode and enter static/residual pressures to project available flow and estimate the color class at 20 PSI.
- Record and use for preplanning. Log the result in your pre-incident plan. Pair with the Fire Flow (NFF) Calculator to confirm the hydrant meets your needed fire flow for the target structure.
Common Use Cases & Limitations
✅ Good uses for this tool
- Pre-incident surveys and first-due water supply checks
- Hydrant marking programs (NFPA 291 color classification)
- Fire academy training — pitot operations and flow estimation
- Engine company drills — water supply decision practice
- Quick field verification of expected hydrant output
⚠️ Important limitations
- Theoretical estimate only — real flow varies with main pressure, condition, and demand
- Projection assumes one flowing outlet and simultaneous residual readings; multiple-outlet tests are not supported
- Not a substitute for full NFPA 291 hydrant flow test procedures
- Do not use for ISO submissions without formal testing
- Always follow department SOP/SOG and local water authority data
Hydrant Flow Calculator — Frequently Asked Questions
Q = 29.83 × c × d² × √p,
where d is the outlet diameter in inches,
p is pitot gauge pressure in PSI, and
c is the discharge coefficient.
For a 2½" rounded outlet at 50 PSI: Q = 29.83 × 0.90 × 6.25 × 7.07 ≈ 1,186 GPM.
This formula is referenced in IFSTA Engine Company Operations and NFPA 291.