NFPA 25 calculation brief
How to Read a Fire Hydrant Flow Test
How to Read a Fire Hydrant Flow Test explains the calculation inputs, formula context, field checks, and limits engineers should confirm before using results in a project workflow.
Page role
Use this page for a defined task before moving to the next project record.
- Audience
- Engineers, designers, technicians, contractors, and reviewers using a calculator-linked technical brief.
- Job
- How to Read a Fire Hydrant Flow Test supports hydrant flow and water supply comparison workflow by turning verified inputs into a calculator result, reference check, and worksheet handoff instead of a standalone explanation.
- Evidence
- Static pressure, Residual pressure, Pitot pressure or test flow, Outlet diameter, Required residual pressure
- Output
- Confirm static, residual, pitot, and flow values belong to the same test event. Calculate available flow at the same residual pressure used for demand comparison.
- Next
- Use /worksheets/hydrant-flow-water-supply before treating this topic as complete.
Calculator-first route
- 1Confirm the worksheet context before using this brief as a calculation input.
- 2Verify required inputs and linked reference data before opening the calculator.
- 3Run the related calculator, review warnings, and compare the output against the review checks.
- 4Carry only reviewed values into the worksheet, authority record, or final project package.
Technical brief workflow
Use this page as a calculator-linked technical brief: confirm the role, verify inputs, run checks, and move to the next tool.
| Brief phase | Engineering detail | Evidence or next step |
|---|---|---|
| Calculator role | How to Read a Fire Hydrant Flow Test supports hydrant flow and water supply comparison workflow by turning verified inputs into a calculator result, reference check, and worksheet handoff instead of a standalone explanation. | Hydrant Flow Test Calculator, Available Water Supply Calculator, Water Supply Curve Plotter |
| Inputs to verify | Static pressure, Residual pressure, Pitot pressure or test flow, Outlet diameter, Required residual pressure | Sprinkler Pipe Internal Diameter Reference, US Code Adoption and AHJ Checkpoints |
| Review checks | Confirm static, residual, pitot, and flow values belong to the same test event. | NFPA 25 |
| Next step | Start from hydrant flow and water supply comparison workflow when this topic controls a design or field-review decision. | Use /worksheets/hydrant-flow-water-supply before treating this topic as complete. Open /calculator/water-supply/hydrant-flow-test when the topic needs numeric screening. Check /reference/pipe-dimensions before transferring reviewed values into the project package. |
Input checklist
- 1Static pressure
- 2Residual pressure
- 3Pitot pressure or test flow
- 4Outlet diameter
- 5Required residual pressure
Next steps
- 1Start from hydrant flow and water supply comparison workflow when this topic controls a design or field-review decision.
- 2Use /worksheets/hydrant-flow-water-supply before treating this topic as complete.
- 3Open /calculator/water-supply/hydrant-flow-test when the topic needs numeric screening.
- 4Check /reference/pipe-dimensions before transferring reviewed values into the project package.
Technical review checks
- 1Confirm static, residual, pitot, and flow values belong to the same test event.
- 2Calculate available flow at the same residual pressure used for demand comparison.
- 3Flag stale, incomplete, or mixed hydrant test data before project reliance.
- 4Carry supply results into sprinkler demand or pump worksheets with pressure basis.
Technical explanation
Formula, variables, example, interpretation, and use limits for the calculation topic.
Formula
Q_available = Q_test x ((P_static - P_required) / (P_static - P_residual))^0.54
The flow-test relationship estimates available water at a required residual pressure from a measured hydrant test. It should be used only when static, residual, and flow readings belong to the same test event.
Variables
| Symbol | Name | Unit | Meaning |
|---|---|---|---|
| Q_test | measured test flow | gpm | Flow recorded during the hydrant test. |
| P_static | static pressure | psi | Pressure before flowing water. |
| P_residual | residual pressure | psi | Pressure while the test flow is occurring. |
| P_required | required residual pressure | psi | Residual pressure used for design comparison. |
Available flow from a hydrant test
Assumptions
- 1Static pressure is 70 psi.
- 2Residual pressure is 50 psi at a measured test flow of 1000 gpm.
- 3The project wants available flow at 20 psi residual.
Steps
- 1Compute the test pressure drop as 70 - 50 = 20 psi.
- 2Compute the available pressure drop to 20 psi residual as 70 - 20 = 50 psi.
- 3Divide 50 by 20 and raise the result to the 0.54 exponent.
- 4Multiply by the 1000 gpm measured test flow.
- 5Record the test date, hydrant location, and required residual pressure with the result.
Result
The available-flow screen is about 1640 gpm at 20 psi residual. It is a water supply estimate, not a guarantee that the supply is acceptable for a specific sprinkler demand or pump selection.
Interpretation
- 1Available flow increases as the required residual pressure is reduced.
- 2A demand comparison is meaningful only when supply and demand use compatible pressure basis.
- 3Low margin should trigger engineering, utility, or AHJ follow-up before reliance.
Common mistakes
- 1Mixing static pressure from one test with residual pressure or flow from another test.
- 2Comparing available flow at 20 psi against a demand that needs a different residual pressure.
- 3Ignoring stale test dates, nearby construction, closed valves, or seasonal supply conditions.
- 4Treating pitot flow and available-flow projection as the same result.
Limits and sources
- 1This method depends on field data quality and assumes the test represents current supply conditions.
- 2Private mains, pumps, tanks, closed valves, and municipal changes can invalidate old test data.
- 3Final acceptance should follow project criteria, AHJ requirements, and qualified review.
- 4Use the original flow-test report as the primary record.
- 5Use adopted NFPA 25, NFPA 13, local utility, and AHJ context for final project decisions.
Where this fits in the workflow
Use this brief inside the hydrant flow and water supply comparison workflow. It should answer a calculation handoff question, not act as general reading. Start from the related worksheet and open the linked calculator only after the input source is known.
Inputs to verify
Before running a tool, verify: Static pressure, Residual pressure, Pitot pressure or test flow, Outlet diameter, Required residual pressure. Check units, equipment listings, pipe or conductor data, and whether the value comes from measured field data or an early design assumption.
Calculator sequence
Use the related calculator set for this topic: /calculator/water-supply/hydrant-flow-test, /calculator/water-supply/available-flow, /calculator/water-supply/supply-curve. Review warning states and compare the output with the linked reference basis before transferring it into a worksheet.
Project record handoff
Retain the calculator inputs, result, formula context, warning state, reference row, and worksheet handoff note together. The next project record is /worksheets/hydrant-flow-water-supply.
Limits before project use
Confirm static, residual, pitot, and flow values belong to the same test event. Treat the output as a screening value. If the result controls design, has a narrow margin, affects equipment selection, or affects AHJ approval, it should be reviewed in the full design package by a qualified professional.
Worksheet handoff
Calculation tools
Next records
How to Read a Fire Hydrant Flow Test
Open related tools
Before filing
- 1Start from hydrant flow and water supply comparison workflow when this topic controls a design or field-review decision.
- 2Use /worksheets/hydrant-flow-water-supply before treating this topic as complete.
- 3Open /calculator/water-supply/hydrant-flow-test when the topic needs numeric screening.
- 4Check /reference/pipe-dimensions before transferring reviewed values into the project package.