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Tools / Hydrant Flow Test Calculator

Hydrant Flow Test Calculator

Turn your pitot readings into the number a sprinkler designer needs: the flow the water supply can deliver at 20 psi residual. Uses the NFPA 291 pitot formula and the 0.54 projection, then draws the supply curve and checks it against a system demand.

1 · Gauge readings at the residual hydrant

20 psi is the conventional design residual for public water supplies.

3 · Compare with a system demand (optional)

Enter the flow and pressure the system needs at the same location as the test (after any elevation and underground friction). The tool reports the pressure the supply holds at that flow.

2 · Flowing hydrant outlets (pitot readings)

Flow
—
Q = 29.83 · c · d² · √p. Use the coefficient for the outlet you actually have — a smooth, rounded nozzle is 0.90; a projecting one is 0.70 and reads low if you assume 0.90. Aim for pitot readings of 10 to 30 psi (NFPA 291), and use 2½-in. outlets where you can: the 4½-in. pumper outlet needs a correction this calculator does not apply.
Total flow during test
—
gpm
Pressure drop
—
% of static
Flow available at 20 psi
—
gpm (projected)

How to run the test

  1. Pick a residual hydrant near the building connection and one or more flow hydrants downstream of it on the same main.
  2. With nothing flowing, read the static pressure on the residual hydrant (cap gauge).
  3. Open the flow hydrant(s) fully and let the flow clear. Read the residual at the residual hydrant and the pitot at each flowing outlet (centered in the stream, about half a diameter from the outlet).
  4. Close slowly to avoid water hammer. Record the time, hydrant locations and outlet sizes — a test is only valid for those conditions.
Estimating tool: results depend on gauge accuracy and on demand on the system at the time of the test. Municipal pressures change by hour and season. Confirm design water-supply data with the AHJ and the designer of record; this calculator does not replace the engineer’s hydraulic calculations.

Basis and further reading