Hydrant flow test calculator

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. A test is most reliable when the residual drops at least 25% 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.

How the hydrant flow test works

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 Q_F is the flow measured during the test, h_r is static minus the target residual (static − 20), and h_f is static minus the observed residual. Example: static 70, residual 50, flowed 1,000 gpm → h_r = 50, h_f = 20 → 1,000 × (50/20)^0.54 ≈ 1,640 gpm at 20 psi.

Reading flow from a pitot gauge

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.

NFPA 291 hydrant color classification

Why 20 psi residual?

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.

Worked example

Static 62 psi, residual holds at 48 psi while a single 2½″ outlet flows an 11 psi pitot:

From the hydrant to the panel

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.

Frequently asked questions

How do you calculate available fire flow at 20 psi?

Use Q_R = Q_F × (h_r / h_f)^0.54, where h_r is static minus 20 psi and h_f is static minus the residual observed during flow. It projects the tested flow to a standard 20 psi residual.

How do I convert pitot pressure to gpm?

Q = 29.83 × c × d² × √P. For a 2.5-inch smooth outlet (c = 0.9) at 10 psi pitot, that is 29.83 × 0.9 × 6.25 × √10 ≈ 531 gpm.

What do the hydrant cap colors mean?

Per NFPA 291: light blue is 1,500+ gpm (Class AA), green is 1,000–1,499 (A), orange is 500–999 (B), and red is under 500 gpm (C), all rated at 20 psi residual.

Why must the residual drop during the test?

The projection needs a measurable pressure drop to model the system curve. If the residual barely moves, flow a second or third outlet so the drop is at least about 25% below static; otherwise the available-flow estimate is unreliable.

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, and the hydrant flow test guide. Water-supply competency supports the pump operations JPRs in NFPA 1002.