A field note that says “hydrant at the loading dock, 58 psi” leaves too much unanswered.
The designer needs to know whether that was a static, residual, or pitot reading, which outlet was open, and its inside diameter. Without those details, the note goes back to the field.
Hydrant flow testing for commercial buildings involves field measurements, calculated discharge in US gallons per minute (GPM), and engineering interpretation.
A usable report keeps those categories separate so the designer and the authority having jurisdiction (AHJ) can review the supply data.
Get the right readings before calculating
Coordinate the test. Before anyone opens a hydrant, get the utility’s permission and confirm that testing personnel meet the utility’s and AHJ’s qualifications.
Check that gauges and pitot equipment suit the hydrant and have documented calibration.
Plan where the discharge will drain, protect pedestrians and traffic, and ask whether a utility or fire-marshal representative must witness the test or receive a specific report format.
Settle the test’s purpose, too. WaterOne’s guidance separates a test using two or more hydrants, which measures the capacity of the underground main, from a test of an individual hydrant. Confirm which one the designer needs, since the result feeds the water supply analysis for the building’s commercial fire sprinklers.
Separate the three pressures. Static pressure is read at the residual hydrant before test discharge begins. FLOW’s hydrant-testing guidance describes it as the pressure in the main with no flow.
Here, “no flow” means no test discharge; ordinary demand elsewhere in the distribution system may continue.
Residual pressure comes from the same gauge while the flow hydrant discharges. Pitot pressure is read at the flowing outlet with the pitot tube held in the stream.
Residual and pitot readings must describe the same flowing condition. A residual reading taken after an outlet is partly closed cannot be paired with a pitot reading taken at full discharge. Which hydrant flows and which carries the residual gauge depends on the main layout and the utility’s direction.
Record each outlet’s inside diameter and entrance shape (rounded, square, or projecting) in the field so the coefficient can be checked later.
Calculate discharge one outlet at a time
For one outlet, the equation documented by MTAS is Q = 29.83 × c × d² × √p. Here, Q is discharge in US GPM, c is the dimensionless discharge coefficient, d is the outlet inside diameter in inches, and p is pitot pressure in psi.
Only pitot pressure goes under the square-root sign. Confirm the accepted testing procedure and applicable standards edition with the AHJ.
For bare hydrant outlets, WSRB lists these coefficients:
| Bare outlet entrance | Coefficient c
|
|---|---|
| Rounded | 0.90 |
| Square | 0.80 |
| Projecting into the barrel | 0.70 |
These values apply to bare outlets. For a diffuser or proprietary nozzle, use the manufacturer’s coefficient or flow chart rather than assuming a bare-outlet value applies.
Work one example. Values are illustrative. Take a bare 2.5-inch outlet with a rounded entrance (c = 0.90) and a pitot reading of 16 psi. Then d² = 6.25 and √p = 4. Multiplying, Q = 29.83 × 0.90 × 6.25 × 4 = 671.175, which rounds to 671 GPM. Keep the unrounded figure until the final total.
Once the field readings and outlet coefficient are confirmed, a pitot-to-GPM calculator can check the arithmetic. FLOW’s free calculator accepts these three inputs and returns single-outlet discharge.
Its page also includes a printable chart for a 2.5-inch outlet, with a column for each coefficient listed above.
The calculator relies on entered field readings; its output belongs in a documented flow-test report for the designer and AHJ to review.
Add simultaneous outlets. When two outlets flow at the same time, each needs its own pitot reading, diameter, and coefficient.
In a hypothetical test where both outlets had the inputs above while flowing together, the total would be 671.175 + 671.175 = 1,342.35, rounded to 1,342 GPM.
That total is valid only if both outlets were measured under the same test condition. Opening another outlet changes pressures in the main, so doubling an earlier single-outlet result does not establish the combined discharge.
Turn the calculation into a usable test report
Label measured and calculated values. Outlet discharge Q describes how much water left the open outlets under test conditions. A designer may need flow projected at a stated residual pressure.
MTAS expresses the common projection as Q20 = Qt × [(S – 20)/(S – R)]^0.54, where Qt is total test discharge and S and R are static and residual pressures.
Pavey Flow Solutions describes that available-flow figure as a calculated projection from measured test data.
The 20 psi in that equation is a residual pressure, not a pitot pressure or a universal sprinkler-design acceptance value.
The equation cannot be used when static and residual readings are identical; review the test data rather than extrapolating.
WaterOne presents test results as information about conditions at the time of the test.
The designer and AHJ determine how accepted supply data relate to the building’s hydraulic demand.
Make the handoff traceable. Record these details for the designer’s review:
- Project name, location, date, and time of day
- Hydrant identifiers and which served as residual and flow hydrants
- Test arrangement and who witnessed it
- Static, residual, and pitot readings, each labeled
- Each outlet’s inside diameter, entrance shape, and coefficient
- Gauges and pitot equipment used, with calibration information
- Calculated discharge per outlet and the total, marked as calculated values
- Observed conditions, such as weather, nearby construction, or other demand
Use this checklist alongside the utility’s and AHJ’s reporting requirements. Some jurisdictions require their own documentation.
San Diego’s permit bulletin for private underground fire service and hydrants, for example, requires a Hydrant Flow Request Form (DS-160) for projects within its scope.
