Engine Ops · Water Supply Tool
Fire Hydrant Flow Calculator
Pitot PSI → GPM + NFPA 291 Color Class (Blue / Green / Orange / Red)
Uses the standard fire service formula Q = 29.83 × c × d² × √p. Free, mobile-ready, no login required.
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Enter Pitot Readings
⚠ Training use only. Always follow department SOP/SOG and conduct formal tests per NFPA 291 for official preplanning.
NFPA 291 Color Class
Enter pitot pressure above to see classification
Estimated Flow
—
GPM
Q = 29.83 × c × d² × √p
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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
Class AA
≥ 1,500 GPM
Excellent — multi-line supply
GREEN
Class A
1,000 – 1,499 GPM
Good — residential/commercial
ORANGE
Class B
500 – 999 GPM
Marginal — single line
RED
Class C
< 500 GPM
Inadequate — boost/shuttle needed
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
Result: ≈ 1,186 GPM → Class A (GREEN) — adequate for most residential structure fires.
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 PSI | Est. GPM | NFPA 291 Class | Color | Operational Guidance |
|---|---|---|---|---|
| 10 PSI | 530 GPM | Class B | Orange | Single 2½" line; monitor residual carefully |
| 20 PSI | 750 GPM | Class B | Orange | Good for single attack line; limited backup |
| 30 PSI | 918 GPM | Class B | Orange | Near Class A threshold; verify residual pressure |
| 40 PSI | 1,060 GPM | Class A | Green | Adequate for residential single-family fire |
| 50 PSI | 1,186 GPM | Class A | Green | Good for residential/light commercial |
| 60 PSI | 1,298 GPM | Class A | Green | Supports attack + backup; solid supply |
| 70 PSI | 1,402 GPM | Class A | Green | High Class A; near Blue threshold |
| 80 PSI | 1,498 GPM | Class A/AA | Blue | Borderline Class AA; excellent for most incidents |
| 90 PSI | 1,589 GPM | Class AA | Blue | Excellent; suitable for commercial/industrial pre-plans |
| 100 PSI | 1,675 GPM | Class AA | Blue | Very high flow; verify hydrant and gauge condition |
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. Most field flow tests use the 2½" (steamer) port. Use 4" or 4.5" if testing a large pumper outlet.
- Choose the discharge coefficient. For most modern hydrants with rounded outlets, use 0.90. If you're unsure, 0.90 is the standard default.
- 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. The tool returns estimated GPM and the NFPA 291 color class instantly.
- 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
- Does not account for residual pressure drop or multiple flowing outlets
- 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
The standard fire service formula is
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. NFPA 291 recommends color-coding hydrant bonnets and caps based on available flow: Blue (Class AA) ≥ 1,500 GPM — excellent for multi-company operations; Green (Class A) 1,000–1,499 GPM — adequate for most structural fires; Orange (Class B) 500–999 GPM — marginal, may require relay or shuttle support; Red (Class C) < 500 GPM — inadequate, alternative supply needed. These colors are applied to the bonnet cap and steamer cap; body color varies by municipality.
The coefficient depends on the shape of the hydrant outlet: c = 0.90 for rounded or smooth edges (most modern hydrants); c = 0.80 for square or sharp-edged outlets; c = 0.70 for protruding or poorly finished outlets. When in doubt, use 0.90 — it is the standard default for most fire service applications and is specified in IFSTA and NFA references.
This tool provides a theoretical estimate based on the standard discharge equation. Real-world flow can vary due to main pressure fluctuations, hydrant age and condition, elevation changes, simultaneous demand on the system, and coefficient assumptions. Use results for operational planning and training. For official ISO submissions or formal preplanning data, conduct full flow tests per NFPA 291 procedures and verify with your water authority.
A hydrant producing 1,000+ GPM (Class A / Green) is generally adequate for most single-family residential fires. Commercial or industrial properties typically require Class AA (Blue, ≥1,500 GPM). Low-flow hydrants (Red, <500 GPM) will likely require tanker shuttle operations or relay pumping from a remote supply. Minimum required fire flow varies by occupancy type, construction, and local code — use the Fire Flow (NFF) Calculator to determine your needed flow for a specific structure.
Static pressure is the system pressure with no water flowing — measured at a capped outlet. Residual pressure is the pressure measured at a nearby hydrant while another hydrant is flowing, reflecting the remaining system pressure under demand. Pitot pressure (velocity pressure) is measured at the flowing outlet using a pitot tube inserted into the stream — this is what this calculator uses to estimate GPM. Full NFPA 291 flow tests use both pitot and residual readings to project available flow at a standard 20 PSI residual.
A basic NFPA 291 flow test involves: (1) selecting a flow hydrant and a residual hydrant downstream on the same main; (2) recording static pressure at the residual hydrant; (3) opening the flow hydrant fully and recording pitot pressure at the outlet plus residual pressure at the residual hydrant simultaneously; (4) using the collected data to project available flow at 20 PSI residual. This calculator covers step 3 (pitot to GPM). Consult the full NFPA 291 document and your water authority's protocols for complete test procedures.
Yes — this tool is well-suited for pre-incident planning surveys. Use it alongside the Hydrant Finder to locate nearby hydrants, the Fire Flow (NFF) Calculator to estimate needed fire flow for the target building, and the Friction Loss Calculator to plan your hose lay. For formal ISO-rated preplanning data, supplement with official flow tests per NFPA 291.
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