Tools / Hydrant Flow Test Calculator / Field guide
Hydrant Flow Testing, Explained Step by Step — and Why Each Step Is Done
A pressure gauge, a pitot gauge and a flowing hydrant produce the number that decides whether a building needs a fire pump. This guide assumes no background: it explains what the test measures, shows how to do it step by step, gives the reason behind every step, and teaches you to judge whether a result can be trusted. Every section has a Listen button.
Start here: the whole test in one minute
Imagine the water main under the street as a long drinking straw connected to a big reservoir. If nobody is drinking, the straw is full and under steady pressure: that is the static pressure. If one person drinks hard, the pressure at the far end of the straw falls: that is the residual pressure. How much it falls tells you how strong the straw is.
The test does exactly that with a real hydrant:
- Measure the static pressure with nothing flowing.
- Open a second hydrant and let water flow. Measure the residual pressure (at the first hydrant) and the flow (with a pitot gauge at the second).
- Those two points make a line. Follow it down to 20 psi to find how much water the main can deliver at that pressure.
Why anyone cares
A sprinkler system is designed to need a certain flow at a certain pressure. If the street cannot supply it, the building needs a fire pump or tank. The hydrant flow test is how that is decided — which makes accuracy matter.Why the test exists
A water main is not a tank. Water moving through a pipe loses pressure to friction, so the faster it flows, the lower the pressure falls. A fire pushes demand up sharply. The designer therefore needs to know the street’s supply curve: for each flow, what pressure is left.
Why 20 psi?
Twenty psi is the conventional minimum residual pressure in a public water system while fire flow is drawn. It keeps the pressure well above zero so that contaminated water cannot be pulled back into the pipes, and it is the pressure at which fire flows are quoted. That is why every flow test ends with “gpm available at 20 psi.” Your designer may specify a different target.
Who uses the number
- Sprinkler designers, to size pipe and decide whether a fire pump is needed.
- Plan reviewers and fire marshals, to check the design is supported by the street.
- Water utilities and fire departments, who test public hydrants about every five years under NFPA 291 (2022) to verify capacity and marking.
The core idea: two points make a line
Here is the supply curve for the worked example used throughout this guide: static 72 psi, and with two hydrant outlets flowing 1,609 gpm the residual drops to 58 psi.
The two measured points (blue and amber) define a straight line when the flow axis is drawn on the N1.85 scale. Why 1.85? Friction loss in pipe grows with flow raised to about the 1.85 power. Stretch the flow axis by that exponent and the relationship becomes a straight line, so two points are enough and the line can be followed down to 20 psi (green): about 3,268 gpm in this example.
The honest limit
The further you extend the line beyond the flow you actually tested, the less you can trust it. This is why a decent flow and a decent pressure drop matter so much.Plan the test: roles, equipment and timing
The two hydrant roles
The residual hydrant sits upstream of the flow hydrant on the same main, as close as practical to the building. It is the measuring post: it feels the pressure drop caused by the water leaving further along, and it does not flow. The flow hydrant (or several) is where the water leaves and where the pitot is used. NFPA 291 (2022) says a small main may need only one or two flow hydrants, and a large main as many as eight.
Only one hydrant at the building?
That is common and it does not stop you. The test is about the main, so the hydrants do not have to be on the property: use the nearest public hydrants on the same main, with the residual hydrant between the supply and the flow hydrant. If one hydrant is truly all you have, the single-hydrant method puts the pitot or flow device on one outlet and a pressure gauge on a second outlet of the same hydrant (the Hose Monster video below shows it). The residual is read where the water is moving through the hydrant, so it tends to read low and the result is conservative rather than flattering. Confirm that the designer and AHJ accept it, and ask the water purveyor — most keep recent test records for their hydrants.Equipment and why each item matters
- Cap gauge with a bleed cock (2½-in. thread): reads static and residual. The bleed lets trapped air out so the needle is honest.
- Pitot gauge with an air chamber and bleed: reads the stream. Choose a gauge range that suits the job. Sprinkler Age notes that a 200 psi gauge should not be used on a pitot reading 10 to 30 psi, and that liquid-filled or digital gauges reduce needle vibration.
- Hydrant wrench and diffuser or hose: to open the hydrant and direct the discharge safely.
- Recent gauge calibration: a 2 psi error is a large share of a 14 psi drop.
- Camera, notebook and radios or phones: for the record, and for the “read now” call.
- Eye protection, cones and a discharge plan.
Timing
Demand on the system changes by the hour. A test at a quiet time flatters the supply; a test during ordinary or busy periods gives a more honest number. Sprinkler Age suggests morning and early-evening periods in residential areas; the purveyor can tell you when demand peaks, and also whether booster pumps or storage tanks nearby could change the result. Always note the time.
Steps 1–4: before the water flows — what to do, why, and what goes wrong
Taking this to the street?
Open the Field Photo Guide on your phone or print it: illustrated steps, how to read the gauges and place the pitot, and a ten-shot photo checklist for your record.Get permission and tell the right people
Do this: Call the water purveyor (the utility that owns the hydrants) and agree the date, the hydrants and where the water can safely go. Tell the fire department and, if the street will be affected, whoever controls traffic.
Why: Hydrants are part of a public water system. Opening one changes pressure for neighbours, can stir up sediment in the main, and can look like an emergency. The purveyor also knows which hydrants are on which main, when demand peaks, and whether a pump station, tank or closed valve will affect your result.
If it goes wrong: Testing without notice can be an offence, can alarm the fire department, and can give you a result that is wrong for reasons you did not know about.
Pick the two roles: residual hydrant and flow hydrant(s)
Do this: Choose a residual hydrant as close as possible to the building connection. Choose one or more flow hydrants downstream of it on the same main (water reaches the residual hydrant first, then the flow hydrant). Look at each hydrant for damage before you start and note its outlet sizes.
Why: The residual hydrant is your measuring post: it must feel the full pressure drop caused by the water leaving further along. If it were downstream of the flow hydrant, it would read pressure after the water had already left. Putting the residual hydrant near the building makes the reading represent the supply that building will actually see. NFPA 291 (2022) says a small main may need only one or two flow hydrants, while a large main may need as many as eight.
If it goes wrong: Hydrants on different mains, or a flow hydrant between the residual hydrant and the supply, make the numbers meaningless.
Check the discharge area and pick the outlet
Do this: Look at where the water will go: people, cars, soft ground, storm drains, ice in freezing weather. Identify the outlet you will flow and its size. Use a 2½-in. outlet where you can. Look at the inside shape if you can see it: smooth and rounded, square and sharp, or projecting into the barrel.
Why: A hydrant at full flow throws enormous force and volume. The outlet shape decides the coefficient (0.90, 0.80 or 0.70) in the flow formula, and the outlet size is a squared term in the formula, so a wrong size or shape produces a big error. 2½-in. outlets give a clean, full stream; the large 4½-in. pumper outlet needs a correction for the void in the middle of its stream.
If it goes wrong: If you flow a pumper outlet without the NFPA 291 correction, the flow comes out too high and the supply looks better than it is.
Attach the cap gauge to the residual hydrant and bleed the air
Do this: Remove a 2½-in. cap, thread the cap gauge on, slowly open the hydrant fully, then open the gauge's bleed cock until water spits steadily and close it.
Why: Air trapped in the gauge acts like a spring: the needle bounces and reads wrong. Bleeding it out gives you a stable, honest reading. Opening the hydrant slowly protects the gauge and the main from a sudden surge.
If it goes wrong: A needle that keeps creeping up or bouncing after bleeding usually means more air. Bleed again.
Steps 5–9: take the readings
Read and record the static pressure
Do this: With every flow hydrant still closed, wait until the needle is steady. Read it straight on at eye level and write it down. If the needle moves in a small range, note the high and low and use the average.
Why: Static pressure is the starting point of the supply curve — what the main can hold when nobody is drawing water. Everything that follows is measured against it. Reading straight on avoids parallax (the needle appearing to sit on a different mark when you look from the side).
If it goes wrong: A needle that swings a lot means someone nearby is drawing water, a pump is cycling, or there is air in the gauge. Wait and note it.
Open the flow hydrant fully
Do this: Remove the cap from the outlet you will flow, then open the hydrant slowly and all the way. On a dry-barrel hydrant, practitioners count the turns to full open and back off about a quarter turn so it does not bind; follow the purveyor's method. Let the stream clear and the pressure settle.
Why: A fully open hydrant means the only restriction is the outlet itself, which is what the formula assumes. A half-open main valve adds an extra restriction inside the hydrant and wastes energy, so the result would describe the hydrant, not the water main. Water out of a dry barrel that is not fully open can also leak out of the drain port and erode the ground around the hydrant. Letting the stream clear removes air and debris that would make the gauges jump.
If it goes wrong: Never throttle a hydrant to reduce splash. Open it slowly to avoid water hammer, a pressure surge that can damage pipes and fittings.
Position the pitot in the stream
Do this: Hold the pitot blade so its opening is in the centre of the stream, facing the flow, about half the outlet diameter from the outlet opening (about 1¼ in. on a 2½-in. outlet). Keep the air chamber and bleed on top so air can escape. Hold it steady until the needle settles.
Why: The pitot measures the speed of the water by the pressure it exerts as it is brought to rest in the tube. Water in the centre of the stream moves fastest and most uniformly, and the coefficients in the formula were developed for the tip being about half a diameter out. At the edge or farther out, the reading is lower than the true flow.
If it goes wrong: A bouncing needle means turbulence or air — reposition slightly and bleed the pitot. Do not use a high-range gauge (for example 200 psi) on a pitot; it cannot be read accurately at 10–30 psi.
Aim for a pitot reading between 10 and 30 psi, and a big enough pressure drop
Do this: Check the pitot gauge: NFPA 291 says readings under 10 psi or over 30 psi should be avoided if possible. If it is over 30 psi, open another outlet or hydrant. Then check the residual hydrant's gauge: the pressure should have dropped by at least 10 % from static (NFPA 291, 2022 and later). If not, open more outlets or another flow hydrant.
Why: Under 10 psi the needle is almost impossible to read accurately; over 30 psi the blade is hard to hold and can break. The pressure drop matters even more: the projection to 20 psi extends a line drawn through your two points. If those points are close together, a one- or two-psi gauge error swings the answer massively (see the demonstration below). A larger drop anchors the line.
If it goes wrong: If the drop is small and you cannot add flow, state that clearly in the report. The result is only a rough estimate.
Read the residual gauge and every pitot at the same moment
Do this: One person reads the residual (cap) gauge at the residual hydrant while another reads the pitot at the flow hydrant, at exactly the same time. Repeat for each flowing outlet. Take the readings more than once and average if the needles move.
Why: Pressure and flow change from second to second as the system responds. Readings from different moments do not describe the same state of the water main, so the pair (flow, residual) must be simultaneous.
If it goes wrong: A reading taken after the other gauge has drifted pairs a flow with the wrong pressure. Use radios or phones to call "read" together.
Steps 10–11: close out, record and stay safe
Close the flow hydrant slowly, then re-check static
Do this: Close the flow hydrant slowly. Then read the static pressure again at the residual hydrant and compare it with your first reading.
Why: Closing a hydrant quickly stops a moving column of water suddenly and creates water hammer, a pressure spike that can crack fittings. A second static reading is a sanity check: if it is very different from the first, something changed during the test — a main break, a valve operated, a pump starting — and the test may not be valid.
If it goes wrong: Do not assume nothing changed. Compare the two statics and write down any difference.
Drain, cap, record and photograph
Do this: With the dry-barrel hydrant closed, check that it drains before replacing the cap: practitioners feel for suction at the open outlet until it stops. Replace the caps and chains. Record the date, time, weather, hydrant IDs and locations, outlet sizes and coefficients, all readings, and anything unusual. Take the photographs from the field card.
Why: Water left in a dry barrel can freeze and crack the hydrant. Without date, time and conditions, a flow test cannot be defended or compared with the next one, because the supply changes with demand, season and system changes. Photographs prove which hydrant and which gauges produced the numbers.
If it goes wrong: If it does not drain, tell the purveyor before you leave.
Safety
Wear eye protection and keep people and traffic clear of the stream. A hydrant at full flow can scour ground, flood a street and damage vehicles. Hydrant outlets and caps can release suddenly; stand to the side when opening and closing. Follow the purveyor’s procedure where it differs from anything here.The maths, explained in plain language
Step A: turning a pitot reading into a flow
The pitot gauge does not read flow; it reads the pressure of the moving water (the velocity pressure). Faster water pushes harder. Flow is the speed of the water multiplied by the area of the opening, and a correction for how cleanly it leaves. Put the units together and you get the NFPA 291 formula:
Q = 29.83 × c × d² × √p
Q is flow in gpm; c is the discharge coefficient (about 0.90 smooth and rounded, 0.80 square and sharp, 0.70 projecting into the barrel); d is the outlet diameter in inches (squared because area grows with the square of the diameter); p is the pitot pressure in psi (under a square root because speed grows with the square root of pressure). Some references print 29.84; the difference is negligible. If you use a stream straightener the coefficient is higher, and large pumper outlets need a correction — see the FAQ.Worked example. Static 72 psi, residual 58 psi. Two 2½-in. smooth outlets (c = 0.90) read 24 and 22 psi on the pitot — both inside the 10 to 30 psi range NFPA 291 recommends.
- Outlet 1: 29.83 × 0.90 × 2.5² × √24 = 822 gpm
- Outlet 2: 29.83 × 0.90 × 2.5² × √22 = 787 gpm
- Total test flow: 1,609 gpm. Residual dropped from 72 to 58 psi, a 19.4 % drop — above the 10 % NFPA 291 now calls for.
Step B: projecting to 20 psi
The supply line is followed down to the target pressure. In formula form, with the 0.54 exponent (which is simply 1 ÷ 1.85):
Q at 20 psi = Q test × [ (static − 20) ÷ (static − residual) ] ^ 0.54
= 1,609 × [ (72 − 20) ÷ (72 − 58) ]^0.54 = 1,609 × 3.714^0.54 ≈ 3,268 gpm available at 20 psi.Step C: comparing with what the building needs
To check a demand, find the pressure the supply holds at the demand flow: P = static − (static − residual) × (Q ÷ Q test)^1.85. At 1,000 gpm that is about 66.2 psi, comfortably above a 40 psi demand. Remember the test hydrant and the building connection are at different heights: water gives up about 0.433 psi for every foot it rises, so a hydrant 30 ft below the riser means about 13 psi less at the riser.
Why the pressure drop matters so much
The projection draws a line through two points and extends it. If the points are close together, the line’s angle is determined by tiny differences — and a one-psi gauge error changes the angle a lot. The table shows the same one-psi error on two tests.
With a small drop the answer swings enormously on a one-psi reading error; with a good drop it barely moves. That is the reason NFPA 291 asks for a 10 % drop (the 2019 edition said 25 %), and why you open more outlets until you have one. It is also why calibrated gauges, steady needles and averaged readings matter.
Reading the result: what to look for
The calculator will project from any numbers you give it. Whether the result deserves trust depends on the test. Check these before relying on a figure:
Reassuring
A clear pressure drop
At least 10 % between static and residual (NFPA 291, 2022 and later; 25 % in the 2019 edition). A bigger drop anchors the projection to something measurable.
Red flag
A tiny drop
Static 70, residual 68 tells you almost nothing. Open more outlets or another hydrant and retest.
Reassuring
Pitot readings between 10 and 30 psi
NFPA 291 says to avoid readings under 10 or over 30 psi if possible. Inside that range the readings are easiest to make accurately.
Look closer
Projection far beyond what was flowed
If the answer is several times the flow you actually tested, treat it as a rough estimate. Flow more water, or have the designer accept the lower figure.
Look closer
Time of day and season
A quiet-hour or winter test flatters the supply. Write the time down and consider whether it represents busy periods.
Look closer
Booster pumps, tanks and closed valves
These can change results a lot and are often found only when the data is analysed. Ask the purveyor, and compare with earlier tests.
Red flag
Residual above static
Impossible in a real test: a gauge is wrong, the hydrant was not flowing yet, or the readings were not taken together. The calculator rejects it.
Red flag
A pumper outlet used without correction
Using a 4½-in. outlet without the NFPA 291 correction overstates the flow and flatters the supply.
Look closer
Elevation difference
Add or subtract about 0.433 psi for every foot the test hydrant sits above or below the sprinkler connection.
Reassuring
Steady, repeatable needles
A pitot needle that holds still and a residual that settles are signs of a clean test. A bouncing needle points to turbulence, air or a partly closed valve.
What a weak result can mean
- Low static pressure: the hydrant may be on a higher-elevation or lower-pressure zone, or a valve upstream may be partly closed.
- A big drop for a modest flow: the main may be small, long, dead-ended, or restricted by deposits or a partly closed valve.
- A supply that has sagged since the last test: demand growth, deposits in old pipe, or a changed system.
These are common first suspects, not diagnoses. The purveyor can confirm what is on that main.
Mistakes that quietly ruin a test — and why they matter
- Assuming c = 0.90 for every outlet. The coefficient multiplies the flow directly; a projecting outlet read as 0.90 overstates flow by about 30 %.
- Holding the pitot off-centre or too far from the opening, which reads the slower edge of the stream.
- Reading residual and pitot at different moments, so the pair does not describe one state of the system.
- Throttling the flow hydrant. It adds a restriction inside the hydrant, so the result describes the hydrant rather than the main.
- Pitot readings over 30 psi or under 10 psi. They are hard to read or hold accurately.
- Flowing a 4½-in. pumper outlet without the correction. It overstates the supply.
- Wrong gauge range or uncalibrated gauges. A 200 psi gauge on a pitot cannot be read to the nearest psi; a 2 psi error is a big share of a small drop.
- Mixing up the diameter. Diameter is squared, so 2½ in. versus 4½ in. changes the formula by more than three times.
- Closing the hydrant fast, or leaving a dry barrel undrained. Water hammer and freeze damage.
- No date, time, hydrant IDs or photographs, so the number cannot be defended or compared later.
What it looks like in the field
Watch it done
Three videos that match the topic, from different angles. Watch the procedure first, then the one on why more flow gives a better test, then the plotting class.
Watch · 2 min
How to Conduct a Single Hydrant Flow Test | Hose Monster University
A quick visual of the equipment and the single-hydrant procedure. This is a manufacturer video, so it shows their product — the method itself applies to any equipment. Courtesy of TheHoseMonsterCompany · open on YouTube
Watch · 11 min
What's the Advantage of More Flow During a Hydrant Flow Test?
Why flowing more water gives a better projection, from a fire protection engineering team. The description also records the move from a 25 % to a 10 % residual drop between the 2019 and 2022 editions of NFPA 291. Courtesy of MeyerFire · open on YouTube
Watch · 28 min
Hydrant Flow Test Procedure | Plotting Hydrant Flow Data | NFPA
A full class: how water supplies are classed under NFPA 13, the test procedure, and plotting the data on hydraulic graph paper both by hand and with software. Watch it once you have the basics. Courtesy of Irfan CFPS · open on YouTube
Check yourself
Check yourself
10 questions
Glossary
- Water main
- The large underground pipe in the street that supplies hydrants and buildings. The test measures how this pipe behaves, not the hydrant itself.
- Static pressure
- Pressure in the main with no water flowing. Read at the residual hydrant before any flow hydrant is opened.
- Residual pressure
- Pressure at the residual hydrant while the flow hydrant(s) are discharging. It falls as flow rises.
- Pitot pressure (velocity pressure)
- Pressure measured at the centre of the stream leaving an outlet. It converts to flow with the nozzle diameter and coefficient.
- Pitot gauge
- A tube with a small opening that faces into the stream, connected to a pressure gauge, with an air bleed to remove trapped air.
- Cap gauge
- A pressure gauge on a cap that screws onto a hydrant outlet, with a bleed cock, used to read static and residual pressure.
- Discharge coefficient (c)
- A correction for how cleanly water leaves the outlet: about 0.90 smooth and rounded, 0.80 square and sharp, 0.70 projecting into the barrel.
- Residual hydrant
- The hydrant where the cap gauge is attached to read static and residual. It is upstream of the flow hydrants and does not flow.
- Flow hydrant
- A hydrant opened to discharge water so the test has flow to measure.
- Supply curve
- The relationship between flow and the residual pressure the main can hold. Drawn as a straight line on N^1.85 axes.
- gpm / psi
- Gallons per minute and pounds per square inch, the units used throughout NFPA 13 hydraulic calculations.
- Purveyor
- The water utility that owns and operates the hydrants and mains. Always contact them first.
Frequently Asked Questions
What if the building has only one fire hydrant?
The test is about the water main, not the building, so the hydrants do not have to be on the property: use the nearest public hydrants on the same main. If only one hydrant is practical, a single-hydrant test puts the pitot or flow device on one outlet and a pressure gauge on a second outlet of the same hydrant to read the residual. The residual is then read where the water is moving through the hydrant, so it tends to read low and the result tends to be conservative. Confirm that the designer and the authority having jurisdiction accept it, and ask the water purveyor for recent test records on the same main.
Why is the answer given at 20 psi?
20 psi is the conventional minimum residual pressure in a public water system while fire flow is drawn: it keeps pressure well above zero so contaminated water cannot be pulled back into the pipes, and it is the pressure at which fire flows are expressed. Designers therefore need to know how much flow the main can deliver while still holding 20 psi. The calculator lets you change the target if your designer asks for something different.
What does the 1.85 exponent mean?
Friction loss in a pipe grows with flow raised to about the 1.85 power (the Hazen-Williams relationship). If you plot pressure against flow on a scale where the flow axis is stretched by that exponent, the supply curve becomes a straight line, so two measured points define it and it can be extended to 20 psi. The 0.54 in the projection formula is simply 1 divided by 1.85.
How big does the pressure drop have to be?
NFPA 291 (2022 and later editions) calls for at least a 10 % drop between static and residual pressure. The 2019 edition said 25 %, and some utilities and older references use a fixed drop such as 10 psi. A bigger drop means the projection leans on a real, measurable change instead of on small gauge errors. If your drop is too small, open more outlets or more hydrants.
What pitot pressure should I aim for?
NFPA 291 says pitot readings under 10 psi and over 30 psi should be avoided if possible. Under 10 psi the reading is hard to make accurately; over 30 psi the pitot blade is hard to hold and can be damaged. If you are over 30 psi, open another outlet or hydrant so the same flow is shared.
Can I flow the big 4½-inch pumper outlet?
It can be done, but NFPA 291 requires a correction for the void in the large stream, and the pitot reading is not as representative as on a 2½-in. outlet. Using the pumper outlet without the correction overstates the supply. Where you can, use 2½-in. outlets. The calculator does not apply the pumper correction and warns you if you choose that outlet.
How recent does a hydrant flow test have to be?
There is no single national answer for a sprinkler design. Designers and AHJs often want a test from the last year or so, and some publish their own limit. Demand on the system changes by time of day and over the years, so older data is less reliable. Ask the AHJ that will review the plans. Separately, NFPA 291 (2022) says public hydrants should be flow tested every five years to verify capacity and marking.
Who is allowed to run the test?
Hydrants belong to the water purveyor, so many utilities run the test themselves or require their staff or a permit. Fire protection contractors, engineers and fire departments also run tests. Contact the purveyor first: opening a hydrant without permission can be an offence and can disturb the system.
Sources and further reading
- NFPA 291, Recommended Practice for Fire Flow Testing and Marking of Hydrants (NFPA)
- QRFS — NFPA guidance on hydrant testing (summarises the 2022 edition: 10 % drop §4.4.6, number of flow hydrants §4.4.8–4.4.9, five-year testing §4.15.1)
- MeyerFire — What’s the Advantage of More Flow During a Hydrant Flow Test? (the 25 % to 10 % change between the 2019 and 2022 editions)
- Sprinkler Age — Conducting Flow Tests (pitot 10–30 psi, gauge ranges, timing, pumper outlets, dry-barrel operation)
- MWUA — NFPA 291 Hydrant Flow Testing overview (pitot readings to avoid, air chamber elevated)
- Municipal Technical Advisory Service (University of Tennessee) — Conducting a fire flow test (hydrant roles, step order, re-check static)
- Fire Engineering — Fire Flow Testing (pitot placement one-half nozzle diameter from the outlet)
- LifeSafetyWiki — Hydraulic Calculations, Fire Hydrants and Hydrant Annual ITM articles
Accuracy and limits
This guide explains the method as described in NFPA 291 and the practitioner sources above. Section numbers and some thresholds differ between editions (for example the minimum pressure drop is 10 % in the 2022 edition and 25 % in 2019), and some references list slightly different outlet coefficients, so check the edition your purveyor and AHJ follow. The authority having jurisdiction and the designer of record decide what water-supply data is acceptable for a project.Run your own numbers in the calculator → · Field photo guide → · Next: reading a fire pump test →