Free Engineering Tool · NFPA 291
Hydrant Flow Test Calculator
Turn hydrant flow test readings into a water supply. Enter static and residual pressure and the pitot reading at each open outlet; the calculator totals the flow, projects the flow available at a target residual (20 psi by default) and plots the supply curve on N1.85 paper, following NFPA 291.
- Based on NFPA 291
- Free, runs in your browser; your inputs stay on your device
- Screening tool, not reviewed or sealed by a licensed engineer
- Last updated October 2026
Screening tool, not engineering advice. This calculator estimates results with simplified published methods. It has not been reviewed or sealed by a licensed engineer for your project, and DGA does not warrant that its results are accurate, complete or suitable for any use. Check every result independently and have the licensed professional responsible for your project review it before you rely on it. Use is subject to the Terms of Use.
Test Information
Static and Residual Pressure
Flowing Outlets
| # | # Outlets | Orifice Size (in) | Outlet Type / Coefficient | Pitot (psi) | Flow per row (gpm) | Comments |
|---|
Available Flow
Water Supply Graph
About This Tool
A hydrant flow test records static pressure at a residual hydrant, then opens one or more flow hydrants and records the residual pressure while they flow, along with the pitot (velocity) pressure at each open outlet. Each pitot reading becomes a flow with Q = 29.84 · c · d2 · √p, and the outlets are added to give the total test flow.
The available flow at another residual pressure is projected with the Hazen-Williams relationship: Qtarget = QF · ((PS − Ptarget) / (PS − PR))0.54. The tool applies the NFPA 291 pumper outlet coefficients, checks that the test produced at least a 10 percent pressure drop, and draws the static, test and projected points on N1.85 paper, where the supply plots as a straight line.
When to bring in an engineer
A flow test is the starting point for sprinkler, standpipe and fire pump design, but the raw numbers rarely go straight into a design. Tests taken on a dead-end main, at a different elevation than the riser, or during a low-demand hour need judgment before they are used. DGA reviews flow test data and builds the water supply analysis as part of fire pump design and sprinkler hydraulic calculations.
Frequently Asked Questions
How do you calculate flow from a pitot reading?
Flow in gpm equals 29.84 times the outlet coefficient times the outlet diameter squared (in inches) times the square root of the pitot pressure (in psi). For a 2.5 inch outlet with a coefficient of 0.90 and a 16 psi pitot reading, that is about 671 gpm.
Why is available flow reported at 20 psi?
Fire flow is defined as the flow available at 20 psi residual pressure, the pressure kept in the main so that it stays positive while fire department pumpers draw from it. Sprinkler and standpipe design use the whole supply curve, compared against the system demand.
What if the pressure drop during the test is less than 10 percent?
NFPA 291 looks for a drop of at least 10 percent of static pressure so the projection is reliable. With less drop, open more outlets or hydrants. Where automatic pumps hold the pressure up, the tool can assume an artificial 10 percent drop to build a theoretical curve, as NFPA 291 allows.
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Engineering aid only, provided as is and without warranty. Results depend on the inputs and on the simplifying assumptions of the method summarized on each tool page, and DGA has not verified them for your application. They do not replace code-required design, testing or review by the licensed engineer responsible for the project. Use of the tools is subject to the Terms of Use.
Related Reading
Articles from DGA Insights on the code and the method behind this calculator.