Water Supply · Process

How Is a Hydrant Flow Test Run and Read (NFPA 291)?

18 min read · Written by DGA Fire Protection Engineering

Reviewed by a licensed Professional Engineer · NY, NJ, PA, MD, DE · Last reviewed November 2026 · Code basis: NFPA 291 (2022) Ch. 4; NFPA 13 (2022) Ch. 4, 5 and 28; AWWA M17; NYS 2025 Uniform Code; NYC 2022 Building Code; NJ UCC 2024 I-Codes; PA UCC 2021 I-Codes; MD 2021 IBC, NFPA 1 and 101 (2024); DE Reg. 701 (2026), NFPA 13 (2025)

General guidance, not a code determination for any project. Confirm the adopted code and local amendments with the authority having jurisdiction.

A hydrant flow test is run by opening one hydrant to flow water while a gauge on a second hydrant on the same main records the pressure, and it is read by converting the pitot reading at the flowing outlet into gallons per minute and projecting that result to the flow the main could deliver at 20 psi. The procedure comes from National Fire Protection Association (NFPA) 291, Recommended Practice for Water Flow Testing and Marking of Hydrants (2022 edition), Chapter 4. The three numbers a sprinkler designer needs from the test are the static pressure, the residual pressure and the flow that produced the drop, because together they define the water supply curve that every hydraulic calculation under NFPA 13 is plotted against.

The test itself takes about an hour in the street. Getting permission to run it, finding hydrants that actually represent the site, and documenting the result so a plan examiner accepts it is where projects lose weeks. This article walks through the request, the field steps, the arithmetic, the paperwork each jurisdiction expects and the reasons a flow test gets rejected.

At a Glance: Hydrant Flow Test

Who files: the sprinkler contractor or the fire protection engineer requests the test; the water purveyor’s crew usually operates the hydrants, with the fire department present where local rules require it.
Where it is filed: with the water purveyor, not the building department: New York City Department of Environmental Protection (DEP) through its Permit and Review Information System (PARIS); New Jersey American Water or the local municipal utilities authority (MUA) in New Jersey; the municipal water authority in Pennsylvania; Washington Suburban Sanitary Commission (WSSC) Water or the county utility in Maryland; the local water utility in Delaware, with the data then going to the State Fire Marshal.
Documents required: a test request or permit naming the hydrants, a flow test report with date, time, location, hydrant identification, static, residual and pitot readings, outlet size and coefficient, gauge calibration records, and a supply curve plotted on N^1.85 graph paper.
Timeline range: 1 to 4 weeks from request to test as of November 2026, longer in winter where purveyors suspend flowing, plus 1 to 3 days to reduce the data and plot the curve.
Most common rejection: a test older than 12 months at the time of the sprinkler submittal.
Approval document: the signed flow test report and curve, accepted by the authority having jurisdiction (AHJ) as the water supply basis on the hydraulic calculations and shop drawings.

How a Hydrant Flow Test Is Run, Step by Step

NFPA 291 (2022) Chapter 4 describes the test as a two-hydrant exercise: one hydrant carries the gauge, one or more hydrants flow. The hydrant with the gauge is the test hydrant, sometimes called the residual hydrant, and it should sit between the flowing hydrant and the source of supply so the gauge reads the pressure the main will actually have at the building. The steps below follow that order from the first phone call to the plotted curve.

Request permission from the purveyor. Every public hydrant belongs to a water utility, and opening one without a permit is tampering. The request names the site, the hydrants proposed, the preferred date and who will attend. In New York City the request goes to DEP through PARIS; in much of New Jersey, Maryland and Pennsylvania the purveyor commonly sends its own crew, usually for a fee.
Pick the test and flow hydrants on the same main near the site. Both hydrants should be on the main that will feed the building’s fire service, in the same pressure zone, and not on a dead-end branch unless the building is on that branch too. The flow hydrant goes downstream of the test hydrant relative to the supply direction, as close to the site’s tap location as the street layout allows.
Record the static pressure. With all hydrants closed, a calibrated gauge on a 2.5 in. outlet of the test hydrant reads the pressure in the main under ordinary demand. NFPA 291 (2022) Chapter 4 recommends testing during normal high-demand periods, with daily and seasonal swings considered, because a night test overstates what the main can deliver on a weekday afternoon.
Flow and record the residual and pitot readings. The flow hydrant is opened fully. Once the stream steadies, a pitot tube held in the center of the stream, about half the outlet diameter from the opening, reads the velocity pressure, and at the same moment the gauge on the test hydrant reads the residual pressure. NFPA 291 (2022) Chapter 4 asks for enough flow to drop the residual at least 10 percent below static (the 2019 edition said 25 percent); American Water Works Association (AWWA) Manual M17 sets the floor at a 10 psi drop, and many reviewers apply both. A smaller drop makes the projection unreliable.
Compute the flow. Each flowing outlet is converted to gallons per minute with the discharge formula below, using the outlet diameter and a coefficient for the outlet shape: 0.9 for a smooth, rounded outlet, 0.8 for a square-edged outlet, and 0.7 for an outlet that projects into the hydrant barrel (NFPA 291 (2022) Chapter 4).
Project the available flow at 20 psi. The measured point is extrapolated along a Hazen-Williams supply curve to the flow the main could deliver while holding 20 psi residual, which is the pressure most purveyors and fire codes treat as the minimum for a public main.
Plot the supply curve. The static point at 0 gpm, the test point and the 20 psi point are plotted on N^1.85 hydraulic graph paper, where a Hazen-Williams supply curve becomes a straight line. The sprinkler demand point, including hose allowance, is plotted on the same sheet.
Document the test. The report records who ran it, the date and time, the weather, each hydrant’s location and identification number, outlet sizes, coefficients, every gauge reading, gauge calibration dates and the elevation difference between the test hydrant and the building’s base of riser.

Figure 1: From Request to Accepted Flow Test

Purveyor scheduling takes 1 to 4 weeks as of November 2026 and the field test about 1 hour; the office work of computing flow, projecting to 20 psi and plotting the curve takes 1 to 3 days. NFPA 291 (2022) Chapter 4 asks for a residual drop of at least 10 percent, AWWA M17 for at least 10 psi, and the test has to be within 12 months of the sprinkler submittal to satisfy most AHJs.

What the Two-Hydrant Setup Looks Like

The physical arrangement decides whether the numbers mean anything. The gauge hydrant must be upstream of the flow hydrant with respect to the supply, so the water that reaches the flow hydrant passes the gauge first and the residual reading reflects the friction loss the building’s service will see. If the hydrants are reversed, or if one sits on a 6 in. dead-end stub while the building taps a 12 in. loop, the curve belongs to a different water supply.

The pitot tube is the only flow instrument in the test. A 2.5 in. outlet flowing full with the pitot held at the center of the stream gives a velocity pressure that, with the outlet coefficient, converts to gallons per minute. On hydrants with two 2.5 in. outlets or a 4.5 in. pumper outlet, each flowing outlet gets its own pitot reading and the flows are added. Flow diffusers with built-in gauges and factory coefficients are common with purveyor crews and are acceptable when the coefficient is recorded.

Figure 2: Gauge Hydrant Upstream, Flow Hydrant Downstream

The gauge hydrant sits between the supply and the flow hydrant, as close to the building’s tap as possible, so the residual reading carries the friction loss the fire service will see. In this test the static is 68 psi, the residual 52 psi and the pitot 18 psi on one smooth 2.5 in. outlet with a 0.9 coefficient, a 16 psi (23.5 percent) drop.

How the Readings Become a Supply Curve: A Worked Example

Two formulas turn three gauge readings into a water supply curve. The first converts the pitot reading to flow. The second extrapolates the measured point to 20 psi along a Hazen-Williams curve, whose exponent of 1.85 inverts to 0.54. Both come from NFPA 291 (2022) Chapter 4, and the second is the Hazen-Williams relationship that the Society of Fire Protection Engineers (SFPE) Handbook, 5th ed., uses for water supply curves; the equations are given here in DGA’s words with the source named.

Flow from the pitot reading: Q = 29.84 × c × d² × √p, where Q is in gpm, c is the outlet coefficient, d is the outlet diameter in inches and p is the pitot pressure in psi.

Available flow at 20 psi: Q₂₀ = Q_f × [(P_s − 20) ÷ (P_s − P_r)]^0.54, where Q_f is the measured flow, P_s the static pressure and P_r the residual pressure.

Take the test in the figure above: static 68 psi, residual 52 psi, one 2.5 in. smooth outlet with a 0.9 coefficient, pitot 18 psi.

Step	Arithmetic	Result
1. Outlet area term	d² = 2.5 × 2.5	6.25
2. Velocity term	√18	4.243
3. Measured flow	Q = 29.84 × 0.9 × 6.25 × 4.243	712 gpm
4. Pressure ratio	(68 − 20) ÷ (68 − 52) = 48 ÷ 16	3.00
5. Raise to 0.54	3.00^0.54	1.810
6. Available flow at 20 psi	Q₂₀ = 712 × 1.810	1,289 gpm (about 1,290)

The main in this example delivers about 712 gpm while holding 52 psi, and about 1,290 gpm if drawn down to 20 psi. The 16 psi drop is 23.5 percent of static, well past the 10 percent NFPA 291 (2022) recommends and the 10 psi floor in AWWA M17, so reviewers accept the result as is. DGA’s hydrant flow test calculator runs these six steps from the field readings, adds multiple outlets and applies the elevation correction.

The elevation correction is the step most often skipped. The gauge measured pressure at the hydrant outlet, and the hydraulic calculation needs pressure at the base of the riser. Every foot the riser sits above the gauge costs 0.433 psi, so a base of riser 6 ft above the hydrant lowers both the static and residual by about 2.6 psi before the curve is plotted.

Figure 3: The Supply Curve and the Demand Point

The supply curve from the worked example runs from 68 psi static at 0 gpm through the test point of 712 gpm at 52 psi to 1,290 gpm at 20 psi. A sprinkler demand of 850 gpm including a 250 gpm hose allowance at 38 psi sits about 8 psi below the curve, so this building needs no fire pump; a demand above the curve would.

Which Documents the Test Produces and Who Prepares Each

A flow test produces a short paper trail, and plan examiners check each piece. The table lists the documents, what each must contain and who prepares it. The report form is not standardized across purveyors, and most AHJs accept any form that carries the fields below.

Document	What it contains	Who prepares it
Test request or permit	Site address, hydrants proposed by number or location, requested date, attendees, fee where charged	Sprinkler contractor or fire protection engineer; approved by the purveyor
Flow test report	Date, time of day, weather, test and flow hydrant locations and identification numbers, outlet sizes and coefficients, static, residual and pitot readings, computed flow, elevation of the gauge, names of the crew and witnesses	Purveyor crew or the contractor’s technician in the field; signed by the person who held the gauge
Gauge calibration records	Calibration date and range for each gauge and pitot used, usually within the past 12 months	Whoever owns the gauges, attached by the contractor
Supply curve	Static, test and 20 psi points plotted on N^1.85 graph paper, with the elevation correction applied and the sprinkler demand point including hose	Fire protection engineer or the sprinkler designer
Water supply information on the hydraulic calculations	Source of the data, date of the test, the three pressures and the flow, the location of the test relative to the base of riser, and any safety margin applied	Sprinkler designer; reviewed and sealed by the engineer where the jurisdiction requires it

NFPA 13 (2022) requires the water supply information to appear with the hydraulic calculations: the location and date of the flow test, the static and residual pressures, the flow and who ran it (Chapter 28, plans and calculations). Chapter 5 of the same edition requires the supply to be adequate for the system demand plus hose at the base of the riser. The article on what a sealed sprinkler shop drawing set includes shows where that water supply block sits on the drawings.

How Long a Hydrant Flow Test Takes and What Delays It

As of November 2026, the request-to-test interval across the region DGA serves runs 1 to 4 weeks. New York City DEP schedules through PARIS and typically lands in the 2 to 4 week range; New Jersey American Water and the larger municipal utilities authorities usually schedule within 1 to 3 weeks; smaller Pennsylvania authorities and Maryland county utilities can be faster or slower depending on crew availability. The field work takes about an hour, and reducing the data and plotting the curve takes 1 to 3 days.

Three things stretch that schedule. Seasonal restrictions are the largest: some purveyors in the region suspend or limit hydrant flowing in winter because of icing on streets and sidewalks, so a winter test may wait until spring unless the purveyor grants an exception. Flushing permits are the second: some utilities treat a flow test as a flushing event and require a separate discharge or dechlorination permit before the crew will open the hydrant. The third is hydrant selection, when the hydrants nearest the site turn out to be on a different main or a dead end and the test has to be rescheduled with new hydrants.

A fourth delay is self-inflicted: ordering the test too early. A test run during schematic design is often more than 12 months old by the time the sprinkler shop drawings are submitted, and the AHJ asks for a new one. The right time to order is when the sprinkler design is about to start, with enough lead to cover the purveyor’s queue. DGA’s project timeline estimator places the flow test in the overall permit schedule.

DGA orders and witnesses hydrant flow tests, plots the supply curve and designs the sprinkler and fire pump systems that depend on it across New York, New Jersey, Pennsylvania, Maryland and Delaware. Project briefs get confirmation and preliminary code feedback within one business day. See where DGA works →

Why Hydrant Flow Tests Get Rejected and How to Fix Each One

Plan examiners reject flow tests for a short list of reasons, and each has a fix. Most rejections trace back to the test not representing the supply the building will actually draw on.

Rejection reason	Why it matters	Fix
Test older than 12 months	Mains, pressure zones and demand change; NFPA 13 (2022) Section 4.5.1.1 limits the test to 12 months before working plan submittal unless the AHJ approves otherwise, and most AHJs enforce that limit	Order a new test timed to the submittal; keep the old one as a comparison
Hydrants on a dead end or a different pressure zone	The curve describes a main the building does not use	Retest with hydrants on the main that will serve the tap, confirmed against the purveyor’s distribution map
No pitot readings	Without pitot pressure and outlet coefficient the flow cannot be verified or recomputed	Record pitot, outlet size and coefficient for every flowing outlet; a diffuser’s gauge counts when its coefficient is noted
Drop less than 10 psi (AWWA M17) or less than 10 percent of static (NFPA 291 (2022) Chapter 4)	A small drop makes the 20 psi projection unreliable and overstates the supply	Flow more water: open a second outlet or a second hydrant and retest
Tested at night or off-peak without adjustment	Night pressures run higher than daytime pressures, so the curve is optimistic	Retest during a normal high-demand period, or apply a documented adjustment from the purveyor’s pressure records
No elevation correction	Pressure at the hydrant is not pressure at the base of riser; each foot of rise costs 0.433 psi	State the gauge elevation and the base of riser elevation, and correct both pressures before plotting
No safety margin	Many AHJs and insurers expect a 5 to 10 psi cushion between the demand point and the supply curve	Plot the margin the AHJ asks for; where the design cannot meet it, the fire pump decision follows

What Each Jurisdiction Expects on the Submittal

NFPA 291 is a recommended practice, not an adopted code, so the enforceable requirement comes from the sprinkler standard the jurisdiction adopted and from the purveyor’s own rules. The table shows who runs or witnesses the test, how to request it and what the reviewer expects to see. Purveyor practice changes often; confirm with the utility before scheduling.

Jurisdiction	Adopted code and AHJ	Who runs or witnesses the test	How to request	What the submittal must show
New York State (outside NYC)	2025 Uniform Code (2024 I-Codes) referencing NFPA 13; local code enforcement official	The local water department or authority; contractor’s technician where the utility allows it	Hydrant use permit from the water purveyor	Flow test report and curve with the hydraulic calculations; many officials ask for the test within 12 months
New York City	2022 Building Code Chapter 9 and Appendix Q (NFPA 13 as modified); Department of Buildings (DOB), with the Fire Department (FDNY) for standpipes and fire department connections	DEP crews conduct the test; FDNY attendance follows DEP’s procedure for the hydrants involved	Online through DEP’s PARIS; fee and scheduling set by DEP	DEP flow test results referenced on the sprinkler or standpipe hydraulic calculations filed in DOB NOW; the test should be recent, 12 months being the usual expectation
New Jersey	Uniform Construction Code, 2024 I-Codes (NJ editions) effective August 17, 2026, referencing NFPA 13; municipal fire protection subcode official	The purveyor’s own crew: New Jersey American Water, Veolia, or the local MUA; contractors generally do not open purveyor hydrants	Purveyor flow test application and fee; 1 to 3 weeks typical	Purveyor-issued flow test letter or report, within 12 months, attached to the fire protection subcode technical section
Pennsylvania	Uniform Construction Code, 2021 I-Codes effective January 1, 2026, referencing NFPA 13; municipal or third-party code official, Licenses and Inspections (L&I) in Philadelphia	The municipal water authority, or the Philadelphia Water Department in the city; some authorities allow the contractor to flow with a witness	Authority hydrant flow test request and fee	Authority report or witnessed contractor report with the hydraulic calculations; 12 month expectation is common but set locally
Maryland	2021 International Building Code (IBC) and NFPA 1 and 101 (2024); county building official and State or county fire marshal	WSSC Water in Montgomery and Prince George’s counties, or the county utility elsewhere; some counties allow licensed contractors to test with a permit	WSSC hydrant flow test request through its permit services; county utility request elsewhere	Flow test data on the sprinkler calculations, with the fire marshal reviewing the water supply basis; confirm the age limit with the county
Delaware	State Fire Prevention Regulations, Regulation 701 (2026), NFPA 13 (2025); Office of the State Fire Marshal (SFM), Wilmington Fire Marshal in the city	The water utility or its approved contractor	Utility hydrant flow test request	SFM plan review expects the flow test data, date and source on the sprinkler submittal before approval; confirm the age limit with the Technical Services Division

Editions as listed in DGA’s Fire Code Cheat Sheet for NY, NJ, PA, MD and DE. Purveyor procedures and fees are not shown and change without notice.

What Happens After the Flow Test Is Accepted

The accepted test becomes the water supply line on the hydraulic calculations, and everything downstream of it depends on that line. The sprinkler designer plots the demand point against the curve, and the gap between them decides whether the system passes on city pressure, needs a larger service, or needs a fire pump. When the demand sits above the curve, the supply curve becomes the suction side of the fire pump calculation, and the pump is sized to lift the available flow to the required pressure; DGA’s guide to the private fire service main covers the underground piping that carries that water from the tap to the riser.

A flow test does not stay valid forever. Two events call for a retest: the design changes enough to move the demand point, such as a storage occupancy replacing an office, or the test passes its age limit before the shop drawings are approved. A system approved on a 12 month old test and built 2 years later has usually been checked against a newer test by the AHJ or the insurer, and NFPA 25 requires the owner to keep tracking the supply after occupancy.

Purveyors also treat flow test data as a snapshot, not a commitment. A letter from the utility saying the main can deliver a given flow at 20 psi describes the day of the test; it is not a guarantee against future zone changes, and most purveyor letters say so. That is why hydraulic calculations carry a safety margin, and why a design that relies on every last psi of the curve is one the plan examiner will question. DGA’s plain-English guide to what NFPA 13 covers explains where the water supply fits in the rest of the standard.

Key Takeaway: A hydrant flow test under NFPA 291 (2022) Chapter 4 is two hydrants on the same main, a static reading, a residual reading taken while the second hydrant flows, and a pitot reading that converts to flow with Q = 29.84 c d² √p. Projecting that point to 20 psi with the 0.54 exponent gives the available flow, and the plotted curve is the water supply every NFPA 13 calculation is measured against. Order it 1 to 4 weeks ahead, test under normal demand, drop the pressure at least 10 percent and 10 psi, correct for elevation and keep it within 12 months of the submittal.

Frequently Asked Questions

How old can a hydrant flow test be for a sprinkler submittal?

Conditional: most AHJs in New York, New Jersey, Pennsylvania, Maryland and Delaware expect the test to be within 12 months of the submittal, which matches NFPA 13 (2022) Section 4.5.1.1. That section lets the AHJ approve older data, and some reviewers accept it with a purveyor confirmation. Order the test when sprinkler design starts, not during schematic design.

Can a sprinkler contractor run a hydrant flow test without the water utility?

No, not on a public hydrant without the purveyor’s permission. In New York City DEP runs the test, in New Jersey the purveyor’s crew runs it, and most Pennsylvania and Maryland utilities either run it or witness a contractor’s test under a permit. Private hydrants on the owner’s main can be tested by the contractor with the owner’s consent.

Why does the flow test project to 20 psi?

Because 20 psi is the residual pressure most purveyors and fire codes treat as the minimum a public main may be drawn down to while fire flow is taken, to keep the main pressurized against contamination. Projecting the measured point to 20 psi gives the available flow at that floor, which is how water supplies are compared.

What pressure drop does a hydrant flow test need?

NFPA 291 (2022) Chapter 4 recommends flowing enough water to drop the residual at least 10 percent below static, and AWWA M17 recommends a drop of at least 10 psi; reviewers commonly apply both. A smaller drop makes the projection to 20 psi unreliable, and the fix is to open a second outlet or hydrant and retest.

Does a hydrant flow test decide whether a building needs a fire pump?

Yes, together with the sprinkler demand. The test produces the supply curve, the hydraulic calculation produces the demand point including hose allowance, and a demand point above the curve means a fire pump or a larger service. A demand point below the curve with the AHJ’s safety margin means the system runs on city pressure.