#370 – NFPA Guidance on Fire Hydrant Testing

Learn when and how cities should perform hydrant flow tests

During a fire emergency, reliable sources of water can mean the difference between life and death. This is why the National Fire Protection Association (NFPA) outlines fire hydrant testing that measures real-world pressure and flow in a city’s water distribution system.

In this blog, we discuss how often flow tests are needed to ensure hydrants will work as expected during a fire. We also dig into the details of what should be done to ensure a water supply meets firefighting requirements.

Need to perform a hydrant flow test? Be sure to browse our full selection of pitot gauge kits, flow test accessories, and pressure gauges.

When does NFPA require fire hydrant testing?

Regular fire hydrant testing ensures the ability to provide water at an acceptable pressure and flow rate for public health and firefighting operations. Most jurisdictions also require hydrant flow tests to design fire sprinkler systems for commercial or residential structures.

The 2022 edition of NFPA 291: Recommended Practice for Fire Flow Testing and Marking of Hydrants (4.2.2) recommends that fire hydrants should maintain a residual pressure of 20 psi (pounds per square inch), or 1.4 bar, for effective firefighting, as well as to prevent backflow that could contaminate the public water supply.

NFPA 291 stipulates hydrant flow tests every five years to ensure that changing conditions in the piping and system demands won’t impede hydrants’ ability to deliver water.

From the 2022 edition of NFPA 291

4.15.1 Public fire hydrants should be flow tested every 5 years to verify capacity and marking of the hydrant.

In the explanation that accompanies the section (A.4.15.1), NFPA clarifies the section’s intent. It states that it does not mean to mandate routine five-year testing of every hydrant—especially if there is no pressing need to test a specific hydrant or if test data less than five years old is available from an adjacent hydrant on the same grid.

Firefighter accessing a hydrant
The middle of a fire emergency isn’t the right time to discover fire hydrants don’t work as expected.

Performed by city officials or professional contractors, fire hydrant testing verifies the performance of a city’s water distribution system, determining the pressure and rate of flow available at various locations. It measures static (non-flowing) and residual (flowing) pressure, as well as the rate of discharge in gallons per minute (GPM) of each fire hydrant.

The data that’s collected is used for two important purposes:

  • Uncovering closed valves, heavy pipe-wall deposits, or other problems in a water distribution system. Reduced rates of flow often stem from blockages or other infrastructure problems.
  • Properly designing fire sprinkler systems for commercial and residential structures. If water supply pressure and flow readings are off, it can lead to an underdeveloped system that requires additional fire pumps or an expensive overhaul of pipe fitting.

Besides delivering peace of mind that hydrants will work in an emergency, hydrant flow tests enable municipalities to color-code their fire hydrants according to their strength of output. The colors categorize hydrants by the GPM of their flow. For instance, the color-coding scheme recommended by NFPA 291 (5.1) and The American Water Works Association says light blue hydrants have a capacity of 1,500 GPM or more (“very good flow”) and red hydrants have a capacity below 500 GPM (“inadequate”).

This system allows fire departments to assess their water resource capabilities quickly when arriving on the scene of an emergency. You can check out our previous blog to learn more about fire hydrant colors and what they mean

Suggested test layouts for fire hydrants diagram
Figure 4.4.4 in NFPA 291 provides several potential layouts for hydrant flow tests.

Preparing to conduct the test

Specific instructions for conducting fire hydrant testing can be found in NFPA 291’s Chapter 4, “Flow Testing.” NFPA 291 guidelines require identifying a residual (test) hydrant to measure static and residual pressure, as well as one or more flow hydrants.

Static pressure represents the pressure at a given point under normal distribution system conditions. It is measured at the residual hydrant with no hydrants flowing. Residual pressure is the pressure that exists in the water distribution system while water is flowing. It is measured at the residual hydrant at the same time flow readings are taken at the flow hydrants.

To determine how many flow hydrants are needed, keep in mind that NFPA 291 recommends flowing enough water to provide at least a 10% drop in residual pressure compared to the static pressure (4.4.6). Further, it states that testers may need to “declare an artificial drop in the static pressure of 10 percent” in “water supply systems where additional municipal pumps increase the flow and pressure as additional test hydrants are opened.” NFPA has updated this guidance from the 2019 edition, which recommended flowing the total demand necessary for firefighting purposes during the test, or enough water to provide at least a 25% drop in residual pressure compared to the static pressure. (In 2018, Sprinkler Age noted that the 25% drop was not necessary for a hydrant flow test used to design a fire suppression system.)

Both editions say that if the mains are small and the system is weak, only one or two hydrants need to flow (2022: 4.4.8/2019: 4.3.7). If the mains are large and the system is strong, as many as eight flow hydrants may be required (2022: 4.4.9/2019: 4.3.8).

Before starting a flow test, it’s important to notify the water company or water authority. Opening a hydrant could disrupt normal operating conditions for the water distribution system in an area.

Testers should also assemble the proper equipment, including:

  • A flow test kit that includes a hand-held pitot gauge to take the pressure and rate-of-flow readings, as well as the correct nozzle size to attach to the hydrants
  • An outlet-nozzle cap outfitted with a pressure gauge that’s used on the residual hydrant
  • A simple ruler for measuring the inside diameter of each flow hydrant’s outlet nozzle
  • A hydrant wrench for accessing hydrants to take residual and flow readings
Pitot gauge kits
Pitot gauge kits like this one make it easy for testers to measure the pressure of moving water during hydrant flow tests.

A water diffuser and sock can also help prevent damage to landscaping and roadways, and redirect water to stop ice patches from forming on certain surfaces in the winter. It’s wise to check that local drains are not blocked by leaves or other debris to prevent water backup. Portable radios can also make testing easier when more than one hydrant flows.

The 2022 edition of NFPA 291 (4.3.1) now suggests conducting hydrant flow tests during periods of “periods of peak demand, based on knowledge of the water supply and engineering judgment,” which is an update from the 2019 edition’s guidance to do it during periods of “ordinary demand” (NFPA 2019: 4.2.1)  That’s likely because many fire protection professionals recommend performing flow tests during peak morning hours to reflect the worst-possible scenario during an emergency. Street pressures can fluctuate as much as 10 psi in the morning, compared to later in the day when demand is typically less.

Be prepared to record the following information during the test:

  • Date of hydrant flow test
  • Location of hydrants being tested (name of the street)
  • Time of day testing was performed
  • Static reading at the residual hydrant (pressure in the system with no flow)
  • Residual reading at the residual hydrant (pressure in the system during flow)
  • Flow reading at the flow hydrant, using a pitot gauge
  • Water main diameter in inches
  • Hydrant outlet size and type (determining the coefficient of discharge)
  • Hydrant elevation

How to conduct a hydrant flow test

Here’s an overview of how to perform a hydrant flow test. Read this carefully: It is not meant as a comprehensive step-by-step guide. Testers should always consult their local authority having jurisdiction (AHJ) and fire department guidelines for the most accurate information.

  1. Determine the location of the test by selecting a group of hydrants in the same vicinity. Remember, as many as eight hydrants may be required for robust systems with large water mains.
  2. Mark the hydrant measuring pressure as the residual hydrant. Both static pressure (when flow hydrants are closed) and residual pressure (when flow hydrants are open) are assessed from this hydrant. The residual hydrant should be between the hydrant(s) to be flowed and the large mains that supply water to the area.
  3. Flush the residual hydrant to remove any sediment and attach a nozzle cap with a gauge to the hydrant’s outlet.
  4. Slowly release the main valve until air is vented. Take a static pressure reading.
  5. Measure the inside diameter of the outlet nozzle or hydrant outlet where flow occurs. A hydrant’s inside diameter is usually 4”.
  6. Field personnel should slowly open each flowing fire hydrant, one at a time, to avoid pressure surges.
  7. After the residual pressure read from the outlet cap stabilizes, take readings at each flow hydrant using a pitot gauge. Residual pressure and pitot gauge readings must be taken simultaneously. For accurate pitot gauge readings, the pitot tube should be held downstream and in the center of the nozzle.
  8. Record both the residual pressure at the residual hydrant and pitot gauge readings at the flow hydrant(s).
  9. Slowly close each fire hydrant.
  10. Use the PSI readings from the residual hydrant’s static and residual pressure, the coefficient determined by measuring the inside diameter of the hydrant’s outlet nozzle, and other factors to determine two sequential numbers using two sequential formulas: the discharge, aka the gallons flowing during the test (gpm), followed by the flow predicted at the desired residual pressure, aka the available fire flow.

We get into the equations of step 10 in the next section, but you can first watch this video to witness a hydrant flow test:

The math needed to assess the results

As mentioned above, the tester needs to gather key information to run two equations in sequence.

The first equation determines the flow (gpm) from the tested fire hydrants based on the pitot gauge pressure readings. A version of it is found in section 4.93 of NFPA 291

Q = 29.83 * c * d2 * √P


Q = discharge; the gallons flowing during the test (gpm)

c = coefficient of discharge, which represents friction loss. It’s determined by assessing the shape of the transition between the vertical barrel of the hydrant and the horizontal outlet. Most hydrants have a smooth and rounded transition resulting in a .90 coefficient of discharge but not all of them (as shown below):

Coefficients of discharge for fire hydrants diagram

d = diameter of the outlet

P = the pressure reading at the pitot gauge during the test (PSI)

You can read a more in-depth look at calculating this gpm value plus access a handy calculator in our previous blog: “Pitot Gauges: How Do I Calculate the PSI to GPM Conversion?

The second formula estimates the “flow predicted at desired residual pressure,” which is sometimes called the “available fire flow” (AFF). This essential equation is found in section of NFPA 291, but here is a version with more steps broken out:

QR = Q * (((S – 20)0.54) ÷ ((S – R)0.54)))


QR = flow predicted at desired residual pressure/available fire flow

Q = the discharge (gpm) measured during the test (the result of the first equation)

S = the static pressure measured during the test

20 = the amount of minimum pressure (in psi) required for most municipal water supplies to prevent backflow and achieve fire protection objectives.

(NFPA 291 calls the “S – 20” calculation above “hr,” which equals “pressure drop to desired residual pressure”)

R = the residual pressure measured during the test

(NFPA 291 calls the “S – R” calculation above “Hf,” which equals the “pressure drop measured during test”)

0.54 = a constant within the Hazen-Williams equation

After conducting a hydrant test, testers plug in their measurements to the two formulas above, completing one after the other.

Fire hydrant testing ensures that hydrants and sprinklers can provide adequate protection

The middle of a fire emergency is not the right time to find out that fire hydrants or sprinklers don’t have enough flow and pressure. Regular fire hydrant testing ensures that this vital equipment works as intended. And when requirements aren’t up to par, these assessments enable repairs to be proactively scheduled so problems can be dealt with before they could lead to loss of life or property.

Ready to perform a hydrant flow test? Be sure to browse our full selection of pitot gauge kits, flow test accessories, and pressure gauges.

If you have questions or need help ordering, call us at +1 (888) 361-6662 or email support@qrfs.com.

This blog was originally posted at blog.qrfs.com. Check us out at Facebook.com/QuickResponseFireSupply or on Twitter @QuickResponseFS.


30 thoughts on “#370 – NFPA Guidance on Fire Hydrant Testing”

      • @stacycantrell2187
        Can someone please help me figure this out? My Static PSI is 40, my Residual PSI is 25, My Orifice inches is 2.5 and my Pitot reading is 20. My waterline is 6 inches and my Coefficient is 0.9….What would my Flow GPM and flow at 20 PSI numbers be…..Please help

        • Bryant — Unfortunately, we can’t recommend fire protection ITM contractors in Arizona. You may want to contact your AHJ or any fire protection associations for references. Best of luck!

      • RR — We publish code and standards interpretations based on the most current documents at the time of writing, and update these periodically as new ones are issued. We will review the latest versions and update when possible — but please be advised that any references in blogs are from editions referenced in a specific post (and enforceable local and state documents may not have been updated along with the latest model codes and standards). Thanks for reading!

  1. When doing a flow test for a future project that is connecting to the city UG, should the residual or flow hydrant be closer to the future point of connection? Is it better to be upstream or down stream? I know every situation is different, but let’s just say this is on a straight line and only 1 flow hydrant is needed.

    • Sonny — For ITM questions like this, you can try our Ask a Fire Pro service. Click the link to submit your question with some information about your building, and a fire protection professional will provide an answer based on best practices, standards, and codes. Our pros include AHJs, contractors, engineers, and code experts with 150+ years of combined experience!

    • Always best practice to have flow hydrant downstream of residual. The closer the residual is to the connection point; the closer the evaluation point is closest to the connection point.

  2. So, outlet of hydrant did not connect any hose with nozzle or nozzle only?
    just fully open the hydrant outlet than check the pressure with pitot gauge, right?

  3. The company I work for conducts their own flow test in conjunction with the Fire Marshal test. Our static/residual were about 30 psi higher than theirs when conducted 5 months later; our GPM was 750 compared to their 1007, however water isn’t the issue. My question is;

    Is there a specific requirement that states a fire marshal must conduct the Flow Test? Can a NICET certify their own flow test? I can’t find anything in NFPA, and the bid set only requires the flow data to be within 1 year of project design.

    • Christopher – For codes and standards interpretation questions like this, you can try our Ask a Fire Pro service. Click the link to submit your question with some information about your building, and a fire protection professional will provide an answer based on best practices, standards, and codes. Our pros include AHJs, contractors, engineers, and code experts with 150+ years of combined experience!

  4. You never grease the threads on the hydrant butts or outlets.the reason is because dirt and dust settles in the grease making the caps more difficult to remove.The threads should be cleaned with wire brush period.No lubricants!

    • Jay — NFPA 291 in the above blog is a “Recommended Practice,” so it has less “you must do this” than a standard, but it specifies testing public fire hydrants every five years and flushing them annually (4.15.1*, 4.15.2). Every five years implies testing at acceptance, in our opinion, since it’s crucial to know whether the hydrant works/delivers flow and pressure. Otherwise, various other NFPA standards for fire protection systems require assessing the water supply, and hydrant testing is often a way to do it.

      Beyond NFPA, there are versions of International Code Council (ICC) codes adopted throughout jurisdictions in the US, including the International Fire Code (IFC), which specifies that ALL system types be acceptance tested; below is from the 2021 edition:

      901.5 Installation Acceptance Testing

      Fire protection and life safety systems and appurtenances thereto shall be subject to acceptance tests as contained in the installation standards and as approved by the fire code official. The fire code official shall be notified before any required acceptance testing.

      Then there are state codes based on these “model” codes, like this bit from The Virginia Statewide Fire Prevention Code (based on the 2018 IFC):

      901.5.2 Hydrant and Fire Service Main Acceptance Testing

      Fire hydrant systems and private fire service mains shall be subject to acceptance tests as contained in the installation standards and as approved by the fire code official. The fire code official shall be notified before any required acceptance testing.

      Bottom line: look for the applicable code in your area and yes, new hydrants should be acceptance tested. Hope that helps and thanks for reading!

  5. Hi,
    I have two questions :
    Q1- I would like to know if in NFPA 291-2022 , it is said that for accuracy of the calculation of flow of fire hydrant, the pitot reading should be between 10 psi and 30 psi .
    Q2- In the last version of NFPA 291 the drop of pressure on the residual was 20% of static pressure. In the 2022 version the drop of pressure was reduced to 10% of residual pressure. why is it like that?

    • Abdelkrim —

      Q1 — We only found a reference to a minimum to avoid inaccurate readings:

      4.8.6 Pitot readings of less than 10 psi (0.7 bar) should be avoided, if possible.

      Pitot readings of less than 10 psi (0.7 bar) can affect the accuracy of the readings and should be avoided.

      Q2 — We assume you are referring to the change in section 4.4.6 (2022) vs. 4.3.6 (2019). We don’t definitively know the committee’s intent without getting and researching the proposal documents. But we can speculate based on the new text (emphasis added to highlight new text):

      4.4.6 To obtain satisfactory test results of theoretical calculation of expected flows or rated capacities, sufficient discharge should be achieved to cause a drop in pressure at the residual hydrant of at least 10 percent. In water supply systems where additional municipal pumps increase the flow and pressure as additional test hydrants are opened, it might be necessary to declare an artificial drop in the static pressure of 10 percent to create a theoretical water supply curve.

      So, in “water supply systems where additional municipal pumps increase the flow and pressure as additional test hydrants are opened,” it seems even achieving/reading even a 10 percent drop has been a common issue. Thus, they may have lowered the number to mitigate this issue and because a 10 percent drop is still sufficient to get satisfactory results.

      If you are an NFPA member or a government/public sector official and would like explicit clarification or more info, you can submit a technical question directly to NFPA! Thanks for reading!

      • Hi,
        I have a question about the The formula used to compute the discharge, Q, in gpm shown in NFP 291 section 4.9.3 equation 4.9.3 a.
        We do fire flow testing using a 2.5” Flowed Hydrant Outlet and we connect to it a Gate Valve , a 45 elbow and a Akron Brass Pitot kit . The formula in the flow chart of Akron Brass is :
        Solid Bore Discharge Formula: 29.71D2 √NP , D = Bore Diameter, NP = Nozzle Pressure Measured with Pitot Gauge (PSI).
        So my question is which formula is accurate ?
        The flow chart of Akron Brass does not show the typical installation to undergo a flow hydrant testing.
        I think that if we use a 45 elbow on the istallation or 90 elbow, the formula to be used will be different than NFPA 291 or Akron Brass flow chart. Thanks !

        • Abdelkrim — We can’t comment definitively and specifically on which is more accurate, however: “C” in the NFPA flow equation accounts for the coefficient of discharge, which varies based on hydrant shape, pointing to some variation in measurements based on equipment. In addition, NFPA regularly defers to “manufacturers’ instructions” for various pieces of fire protection equipment that may have variances from the standards. So, your best course of action is to contact Akron brass and ask them how the NFPA formulas and their formulas reconcile with each other. Thanks for reading!

  6. IF pitot will be LESS than 10 psi
    you can easily get better readings by using a straight Play Pipe,
    and most have different size removable nozzles. This will squirt farther!
    That gives you a clean discharge, and a smaller outlet,
    so the pitot reading will be higher and more accurate.
    Be sure to record and use the new outlet diameter and proper co-efficient.

    Also note the elevation of the hydrants.
    James E. Art
    Fire Protection Engineer
    Pleasanton, Ca

  7. Hello,
    Thank you for all these relevant information.
    Can you estimate the flow of a fire hydrant only with static and residual pressure data? (without pitot gauge) If yes, which formula should beused and can we also use formula found in section of NFPA 291 afterwords to get the flow at 20psi?
    Thank you!

    • NFPA 291 doesn’t have rules on marking hydrants out on the street with date, though it is just a recommended practice document and local jurisdictions may have some sort of system like that. NFPA 291’s main marking recommendations are as follows (2022 edition; and there are some additional sections on this in Chapter 5): The tops and nozzle caps should be painted with the capacity-indicating color scheme shown in Table 5.1 to provide simplicity and consistency with colors used in signal work for safety, danger, and intermediate condition. For rapid identification at night, it is recommended that the capacity colors be of a reflective-type paint. Hydrants rated at less than 20 psi (1.4 bar) should have the rated pressure stenciled in black on the hydrant top. In addition to the painted top and nozzle caps, it can be advantageous to stencil the rated capacity of high-volume hydrants on the top. The classification and marking of hydrants provided for in this chapter anticipate determination based on individual flow test. Where a group of hydrants can be used at the time of a fire, some special marking designating group-flow capacity could be desirable.


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