How to Conduct a Fire Hydrant Flow Test

A fire hydrant flow test — also called a fire flow test — measures how much water a hydrant (and the main behind it) can actually deliver. The number that matters is the available fire flow at 20 psi residual pressure — the flow you can count on while keeping the main above the point where it risks collapse or contamination. Departments use flow-test results for pre-incident planning, engine placement, ISO grading, and sizing an attack before the first line goes to work.

The Three Readings

• Static pressure — the pressure in the main with no hydrants flowing. Read it on a cap gauge at the test (residual) hydrant before opening anything.
• Residual pressure — the pressure remaining at the test hydrant while the flow hydrant is wide open. The drop from static to residual is what reveals the main's strength.
• Flow (pitot) pressure — the velocity pressure measured at the open outlet of the flow hydrant with a pitot gauge, converted to gpm.

You need at least two hydrants: a residual hydrant (where you read static and residual) and one or more flow hydrants (which you open and measure).

Converting Pitot Pressure to Flow

• Q — discharge in gallons per minute.
• C_d — the hydrant outlet coefficient: 0.9 for a smooth, rounded outlet; 0.8 for a square outlet; 0.7 for one that protrudes into the barrel.
• d — outlet diameter in inches (a 2½" outlet is the usual flow point).
• P — the pitot pressure in psi.

Example: a 2½" outlet (C_d = 0.9) reading 10 psi on the pitot flows Q = 29.83 × 0.9 × 2.5² × √10 = 29.83 × 0.9 × 6.25 × 3.16 ≈ 530 gpm.

Projecting Available Flow at 20 psi

You almost never flow enough to actually pull the main down to 20 psi, so NFPA 291 projects it from the test data with the Hazen-Williams exponent:

• Q_R — the projected flow available at 20 psi residual.
• Q_F — the total flow measured during the test.
• h_r — static pressure minus 20 psi (the pressure drop you are allowed to use).
• h_f — static pressure minus the residual pressure you measured (the drop you actually caused).

Worked Example

Static pressure 70 psi. You flow one 2½" outlet and measure 620 gpm, and the residual hydrant drops to 58 psi. Then h_r = 70 − 20 = 50, and h_f = 70 − 58 = 12. Projected flow at 20 psi: Q_R = 620 × (50^0.54 / 12^0.54) = 620 × (8.16 / 3.83) ≈ 1,320 gpm available at 20 psi. That hydrant earns a green classification.

NFPA 291 Hydrant Color Codes

Barrels are generally chrome yellow; the bonnet and caps carry the class color. Always confirm your jurisdiction's adopted edition and any local marking variations.

Safety and Procedure Notes

• Open and close hydrants slowly to avoid water hammer that can rupture mains.
• Flow toward a storm drain or open area; watch for erosion, ice, and traffic.
• Notify the water authority and dispatch — a flow test can drop pressure and stir sediment across a neighborhood.
• Test during average-demand hours; results vary with time of day and seasonal demand.

Frequently Asked Questions

Run the Numbers

Turn a flow-test result into a fireground plan: check what your attack actually needs with the free Required Fire Flow Calculator, size supply against demand with the Water Supply Duration Calculator, and work the discharge side with the Pump Discharge Pressure Calculator.