Find the available fire flow at 20 psi residual from a hydrant flow test using the NFPA 291 formula, then read the color-coded classification. Enter static, residual, and flowed gpm — or start from a pitot reading and this tool converts it for you.
Based on NFPA 291, Recommended Practice for Fire Flow Testing and Marking of Hydrants. The 2022 edition asks for a residual at least 10% below static; too small a drop makes the projection to 20 psi unreliable — flow a second outlet. Estimates for pre-planning and training; verify against your AHJ's procedures and calibrated gauges.
A fire flow test measures how much water a hydrant can deliver at a usable pressure. You read the static pressure with the system at rest, open a nearby hydrant and record the residual pressure that remains while water flows, plus the flow rate discharged. From those three numbers, NFPA 291 projects the flow available at a standard 20 psi residual:
Where QF is the flow measured during the test, hr is static minus the target residual (static − 20), and hf is static minus the observed residual. Example: static 70, residual 50, flowed 1,000 gpm → hr = 50, hf = 20 → 1,000 × (50/20)0.54 ≈ 1,640 gpm at 20 psi.
If you meter flow with a pitot at the outlet instead of a flow meter, convert pressure to gpm with:
Where c is the outlet coefficient (0.9 smooth, 0.8 square, 0.7 projecting), d is outlet diameter in inches, and P is pitot pressure in psi. Set the “flow measured by” field to pitot pressure and this calculator runs that step before projecting to 20 psi. A 2½″ smooth outlet (c = 0.9) at 10 psi pitot flows about 531 gpm.
| Class | Available @ 20 psi | Cap / bonnet color |
|---|---|---|
| AA | ≥ 1,500 gpm | Light blue |
| A | 1,000–1,499 gpm | Green |
| B | 500–999 gpm | Orange |
| C | < 500 gpm | Red |
Twenty psi is the minimum residual pressure the distribution system should retain during maximum flow. Below it, the risk of drawing contaminants into the mains rises and pump intakes can start to cavitate. NFPA 291 standardizes hydrant fire-flow ratings at this residual so hydrants can be compared and marked consistently across a system. The projected number is what a driver/operator can actually count on when laying in.
Static 62 psi, residual holds at 48 psi while a single 2½″ outlet flows an 11 psi pitot:
The flow test tells you what the supply can give. The next question is whether your tactics and pump can use it. Size the demand with the required fire flow calculator, check how long tank water lasts before that supply is established with the water supply duration calculator, and work the discharge side with the friction loss and pump discharge pressure calculators. For the full walkthrough, see the fire hydrant flow test guide.
PumpForge simulates the full pump operation live — supply, intake, discharge pressure, RPM, and flow — across 16 built-in scenarios with automatic scoring, built for NFPA 1002 driver/operator training.
See the fire pump simulator →Formula per NFPA 291 (fire flow testing and hydrant marking). For training and pre-planning; always verify against your AHJ's testing procedures and calibrated gauges. Related: required fire flow, water supply duration, friction loss, the hydrant flow test guide and fire hydrant color codes. Water-supply competency supports the pump operations JPRs in NFPA 1002.