# Pitot Gauges: How Do I Calculate the PSI to GPM Conversion? (2023)

## Follow these simple steps to ensure fire hydrants can deliver adequate water flow during a fire

Firefighting operations can be seriously jeopardized if fire hydrants aren’t ready and able to deliver the required water flow and pressure. Municipalities and other authorities insist upon regular hydrant flow testing to ensure that adequate water is available when it’s needed most—which is where pitot gauges come in.

In theory, pitot gauges make it simple to measure the flow of fire hydrants and convert readings in pounds per square inch (PSI) to gallons per minute (GPM), enabling inspectors to quickly uncover reduced flow rates. But in reality, the equation and chart can be confusing—and QRFS phones are constantly ringing with the question: How do I do this right?

In this blog, we will take you step-by-step through the process of converting a pitot gauge’s PSI reading to GPM. We promise that we’ll make it so simple even your grandmother could do it accurately—especially since we’ve provided a calculator to help you make the conversion. Feel free to jump straight to the formula, followed by a step-by-step process and a calculator.

In the market for pitot gauges? Be sure to browse our selection of pitot gauge kits and accessories, including our best-selling Inspector’s Choice Pitot Gauge Kit, which costs a little more but delivers 1 percent full-range accuracy.

## Pitot gauge 101: here’s how they work

Fire hydrants are color-coded according to their strength of output, enabling fire departments to assess their water resource capabilities quickly when they arrive on the scene of an emergency. For instance, red hydrants typically have a rated capacity of less than 500 GPM and light blue hydrants have a rated capacity of more than 1,500 GPM. With so much at stake, it’s critical for hydrants to perform as expected during a fire.

A handheld pitot gauge is the quickest, simplest method for measuring straight-tip and hydrant-flow GPM. Analyzing the data it collects can also reveal heavy pipe-wall deposits and closed valves that can significantly impede a hydrant’s pressure and flow.

Previously, we’ve offered a detailed guide on how to use a pitot gauge for hydrant flow testing. But here’s a quick overview of how pitot gauges work:

(Video) Converting Hydrant Flow Test Results to GPM

A pitot gauge consists of three components: a blade, handle, and pressure gauge. After it’s inserted into an open fire hydrant’s water discharge, a narrow tube inside the blade directs water toward the gauge to create a pressure reading. Generally, the edge of the blade is set parallel to the direction of fluid-stream movement, at a distance about one-half the diameter of the opening in the center of the stream. Put simply, that means if the orifice is 2.5 inches, the blade should be about 1.25 inches out.

The captured PSI readings from a group of hydrants in the same vicinity are then used to calculate the disparities in pressure between two points along the distribution system. But in order to understand the results, the PSI readings need to be converted to GPM, the standard measurement of water flow. And that’s where it gets a little tricky.

PSI can’t be directly converted to GPM; these are two different units of measure. PSI measures pressure and GPM measures flow rate.

But if other variables are known, the water’s flow rate in GPM can be calculated with the help of the Bernoulli equation for incompressible fluid and careful unit conversion. Bernoulli, a Swiss mathematician, developed a formula that represents the relationship between velocity and pressure along a streamline.

Bernoulli’s original equation must be modified so that it accounts for the friction produced when water flows through a hydrant. When represented mathematically, the friction is called the “hydrant’s coefficient” and is determined by the loss in flow caused by the hydrant’s internal orifice design.

## The formula: what do all those variables mean?

After successfully completing the flow test and recording the information, here’s the formula used to determine the rate of discharge flowing from a single fire hydrant in GPM:

(Video) Hydrant Flow Testing

Let’s break down what those variables mean:

• 29.84 is a constant derived from physical laws relating to water velocity, pressure, and conversion factors. In short, this number keeps the answer in GPM. Some sources offer a slightly smaller constant of 29.83.
• cd = the coefficient of discharge, which represents friction loss.
• d = the actual inside diameter of the hydrant orifice in inches.
• p = the pressure in PSI read at the orifice by the pitot gauge. Because this formula takes the square root of p—rather than p itself—large increases in PSI will have a fairly small impact on the final GPM.
• Q = a number used to represent the result, or discharge in GPM.

Finding the numbers to plug in for “d” and “p” are easy. To determine d, simply measure the inside diameter of the hydrant’s outlet or outlet nozzle where flow occurs; to find p, record the pitot gauge reading.

The discharge coefficient, or “c,” varies with the type of outlet on the hydrant. Different manufacturers have different internal hydrant orifice designs, and friction is created as water is required to go around bends or sharp corners.

In most modern hydrants, the transition between the vertical barrel of the hydrant and the horizontal outlet is smooth and rounded. In the diagram below, consider the difference between the transitions (which look like wings on either side of the middle rectangle) in the first two outlet drawings. The rounded design has a coefficient of 0.9, meaning there is only a 10 percent decline in water flow due to friction loss.

When the transition is square and sharp as in the second drawing, the hydrant has a coefficient of 0.8. When it is square and projects into the barrel of the hydrant like the drawing on the right, the coefficient is 0.7. By poking your fingers inside the outlet, you can determine if it’s smooth and rounded or square and sharp. Compare your findings to the three general types of hydrant outlets in the diagram above to determine the coefficient of discharge.

## Practice solving the equation yourself or with our custom hydrant discharge calculator

Now that you understand where the numbers come from, let’s practice solving the equation. While testing a single fire hydrant, here’s what we found:

• The flow reading on the pitot gauge is 62 psi.
• A single 2.5-inch port is used with a measured inside diameter of 2.55 inches. NFPA 291: Recommended Practice For Fire Flow Testing And Marking Of Hydrants recommends using 2.5-inch outlets for flow tests instead of pumper outlets (which are larger than 2.5 inches) because the larger outlets don’t fill completely and provide inaccurate pressure readings. If pumper outlets have to be used, the resulting flow must be modified to account for the voids in water flow.
• The outlet is smooth and rounded, meaning it has a coefficient of 0.9.

So, what’s the total GPM available at normal operating pressure?

(Video) Flow Testing: Measuring Nozzle Pressure (Episode #86)

Step 1: Multiply the constant 29.84 times the coefficient of discharge (cd). 29.84 x 0.9 = 26.856

Step 2: Square the measured diameter. 2.55 x 2.55 = 6.5025

Step 3: Multiply those two numbers: 26.856 x 6.5025 = 174.6

Step 4: Find the square root of 62 psi, which is 7.8740

Step 5: Multiply the final two numbers to solve for Q, the discharge in GPM. 174.6 x 7.8740 = 1,375 GPM

Now that you’ve got a grasp on how PSI, discharge coefficients, and diameter impact a hydrant’s flow rates, try our handy GPM calculator for fire hydrants:

Watch this video for another step-by-step example of how to use the pitot gauge’s PSI reading to calculate the rate of water flowing from the hydrant in GPM:

(Video) What's the Advantage of More Flow During a Hydrant Flow Test?

## Theoretical discharge tables make converting PSI to GPM even easier

Theoretical discharge tables, like the one we provide with our selection of pitot gauges, make the conversion even simpler. Simply find the pitot pressure reading on the left and slide your finger over to the matching orifice size to reveal the GPM.

But it’s important to keep in mind that the table is based on using a coefficient discharge of 1 in the formula, which is the theoretical discharge from a perfect circular orifice.

In the real world, no orifice is perfect. When more accurate results are required, NFPA 291 explains that the coefficient appropriate for your particular hydrant outlet should be applied to the figures on the table.

In other words, if your pitot pressure is 28 psi and your orifice size is 4, your theoretical discharge according to the table is 2,526 GPM. If the coefficient of your hydrant outlet is 0.9, multiplying 2,526 x 0.9 finds the actual rate of flow: 2,273.4 GPM.

Below is a theoretical discharge table from NFPA 291; it also uses a perfect (impossible) coefficient discharge of 1. Click on the image to see the full-size version:

## Pitot gauges help inspectors ensure that fire hydrants are ready and able to fight fires

Pitot gauges are the fastest and easiest way to measure the rate of discharge from fire hydrants – as long as you understand the proper steps to converting pressure readings in pounds per square inch to gallons per minute. Following these simple instructions helps inspectors deliver peace of mind that fire hydrants will perform as expected during a fire.

(Video) Chapter 09 Lecture on Fire Department Pumper Testing

In the market for pitot gauges? Be sure to browse our selection of pitot gauge kits and accessories, including our best-selling Inspector’s Choice Pitot Gauge Kit, which costs a little more but delivers 1 percent full-range accuracy.

Questions about QRFS products or need a brand or item that’s not in our online inventory? Just call us at +1 (888) 361-6662 or email [emailprotected].

This blog was originally posted at blog.qrfs.com. If this article helped you understand fire hydrant testing calculations, check us out at Facebook.com/QuickResponseFireSupply or on Twitter @QuickResponseFS.

## FAQs

### How do you convert PSI to GPM? ›

To calculate GPM from pressure in PSI for water, follow these steps: Measure the pressure inside the tank using a pressure gauge. Subtract the atmospheric pressure from the tank pressure. Multiply the result from step 2 by 2 and divide by the density of water.

What is the formula for GPM? ›

The formula to find GPM is 60 divided by the seconds it takes to fill a one-gallon container (60 / seconds = GPM). Example: The one-gallon container fills in 5 seconds, breakdown: 60 divided by 5 equals 12 gallons per minute.

Is PSI same as GPM? ›

PSI (Pounds per Square Inch) refers to the amount of cleaning pressure the machine can produce. GPM (Gallons per Minute) is the amount of water that is coming from the machine.

How do you calculate water flow from pressure? ›

To calculate flow rate using differential pressure:
1. Divide the pressure difference (ΔP) by the specific gravity (S) of the fluid.
2. Find the square root of the resultant division.
3. Multiply the root by the flow factor (Kv) to obtain the flow rate (Q) for the fluid system. Mathematically, that's: Q = √(Kv × (ΔP/S))
3 Feb 2022

How many PSI are in a gallon? ›

Pound/gallon (US) to Psi/1000 Feet Conversion Table
Pound/gallon (US)Psi/1000 Feet
1 pound/gallon (US)51.948051948 psi/1000 feet
2 pound/gallon (US)103.896103896 psi/1000 feet
3 pound/gallon (US)155.844155844 psi/1000 feet
5 pound/gallon (US)259.74025974 psi/1000 feet
7 more rows

How do you calculate flow rate with pressure and diameter? ›

Square the pipe's radius. With a radius, for instance, of 0.05 meters, 0.05 ^ 2 = 0.0025. Multiply this answer by the pressure drop across the pipe, measured in pascals. With a pressure drop, for instance, of 80,000 pascals, 0.0025 x 80,000 = 200.

How do you convert psi to velocity? ›

To calculate velocity from pressure, multiply the dynamic pressure by 2, divide by the fluid mass density, then take the square root of that result.

What is gpm flow rate? ›

GPM means Gallons Per Minute. Also known as "flow rate", GPM is a measure of how many gallons of water flow out of your shower head each minute. Since 1992, a maximum of 2.5 GPM is the federally mandated flow rate for new shower heads. This means no more than 2.5 gallons of water should flow out each minute.

What is GPM unit? ›

Gallon per minute – The quantity of water equivalent to a stream which will fill a gallon measure once each minute. A flow of one cfs is approximately equal to either 450 gpm, one acre-inch per hour, or two acre-feet per day (24 hours).

How do you calculate PSI? ›

Find the area of that surface. Make sure that both values are in the correct units, i.e., newtons/meters squared or pounds/inches squared. Divide the force by the area. Your pressure will be expressed in pascals (Pa) or pounds per square inch (psi).

### How do you measure PSI of water? ›

The most accurate method is to buy a pressure gauge from your local hardware store and hook it up to a hose faucet. Check the pressure when all other faucets and water-using appliances are turned off to get a baseline reading. In general, you want the household plumbing to provide between 30 and 80 psi.

How do you calculate the PSI of a pump? ›

The formula used in the calculation process is [(Head(Ft.) X S.G.)/2.31]. Enter the values for head (in feet) and specific gravity of the fluid being pumped to yield pounds per square inch.

How do you calculate flow rate from pressure difference? ›

To find the velocity of the fluid flow, multiply the differential pressure by two and divide this number by the density of the flowing material.

How do you calculate air flow rate? ›

You can also calculate the air flow rate if you already know what the air velocity and pipe diameter are. With this information, you can use the following formula to determine the air flow rate: Qair = uair * π. D2/4.

What is the relationship between pressure and flow rate? ›

The flow rate is proportional to the square root of the pressure gradient (F ∝ √P). To increase turbulent flow twofold, the pressure gradient requires a fourfold increase.

What PSI is 5 gallons? ›

This 5 Gallon air tank with 9 Ports can support up to 200 PSI of air pressure.

How do I convert PSI to volume? ›

V = P1 * V1 / P2
1. Where V is the Volume from PSI (ft^3)
2. P1 is the initial psi.
3. P2 is the final psi.
4. V1 is the initial volume (ft^3)

What is the pressure at 10 feet of water? ›

For example, since water weighs approximately 62.4 pounds per cubic foot, the pressure exerted by 1 foot of water is 62.4 / 144 =. 433 pounds per square inch. Ten feet of water would exert a pressure of 10 X . 433 4.33 pounds per square inch.

How do you calculate the flow rate of a pipe diameter? ›

The equation for pipe diameter is the square root of 4 times the flow rate divided by pi times velocity. For example, given a flow rate of 1,000 inches per second and a velocity of 40 cubic inches per second, the diameter would be the square root of 1000 times 4 divided by 3.14 times 40 or 5.64 inches.

How do you calculate velocity from a pitot tube? ›

For a fluid with known density and measured difference between stagnation pressure and static pressure (ΔP), as measured with a pitot tube, the fluid velocity can be calculated with the equation: V = (2ΔP/ρ)1/2.

### What is gpm per pipe size? ›

Water Flow Capacity in Steel Pipes (sch 40)
Pipe SizeMaximum Flow (gal/min)Head Loss (ft/100 ft)
2"453.9
2-1/2"754.1
3"1303.9
4"2604.0
9 more rows

Does higher GPM mean more pressure? ›

GPM stands for gallons per minute. It refers to the flow rate or the volume of water that moves through the pressure washer's nozzle every minute. Think of GPM as rinsing power. The higher a pressure washer's GPM, the quicker you can wash a surface clean.

How many gallons per minute Should water pressure be? ›

According to federal regulations, all kitchen and bathroom faucets should have a maximum flow rate of 2.2 gallons per minute (gpm) at 60 psi (pounds per square inch) of pressure; in California and Georgia, maximum flow rates are further restricted to 2.0 gpm (California will eventually be restricted to 1.8 gpm).

What GPM is low flow? ›

Unfortunately, there is no firm definition of low-flow, but it's generally accepted that anything using 1.5 gallons per minute (gpm) or less is considered “ultra low flow,” while anything using 2.5 gallons per minute to 1.5 gallons per minute is considered “low flow.” Nowadays, thanks to standards set by the government ...

Is gpm a volumetric flow rate? ›

Another unit used is standard cubic centimetres per minute (SCCM). In US customary units and imperial units, volumetric flow rate is often expressed as cubic feet per second (ft3/s) or gallons per minute (either US or imperial definitions).

What is a good well GPM? ›

The Water Well Board suggests that a minimum water supply capacity for domestic internal household use should be at least 600 gallons of water within a two-hour period once each day. This is equivalent to a flow rate of 5 gallons per minute (gpm) for two hours.

How many cfs are in a GPM? ›

gpm= cfs/448.8325660485.

How do you convert PSI to velocity? ›

To calculate velocity from pressure, multiply the dynamic pressure by 2, divide by the fluid mass density, then take the square root of that result.

How do you calculate the PSI of a pump? ›

The formula used in the calculation process is [(Head(Ft.) X S.G.)/2.31]. Enter the values for head (in feet) and specific gravity of the fluid being pumped to yield pounds per square inch.

How do you calculate PSI? ›

Find the area of that surface. Make sure that both values are in the correct units, i.e., newtons/meters squared or pounds/inches squared. Divide the force by the area. Your pressure will be expressed in pascals (Pa) or pounds per square inch (psi).

### What is the PSI of a fire hydrant? ›

3) 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.

How do you calculate velocity from a pitot tube? ›

For a fluid with known density and measured difference between stagnation pressure and static pressure (ΔP), as measured with a pitot tube, the fluid velocity can be calculated with the equation: V = (2ΔP/ρ)1/2.

How do you calculate air flow rate? ›

You can also calculate the air flow rate if you already know what the air velocity and pipe diameter are. With this information, you can use the following formula to determine the air flow rate: Qair = uair * π. D2/4.

How do you measure PSI of water? ›

The most accurate method is to buy a pressure gauge from your local hardware store and hook it up to a hose faucet. Check the pressure when all other faucets and water-using appliances are turned off to get a baseline reading. In general, you want the household plumbing to provide between 30 and 80 psi.

What is PSI in pressure pump? ›

As we work with water pumps, we find that pressure is presented to us in two common units: PSI (pounds per square inch) or feet of head. As we size a pumping system, we'll want to accomplish building pressure (PSI). The relationship between PSI and feet of head is that 2.31 feet of head = 1 PSI.

How many feet of water are in a PSI? ›

NOTE: One Pound of pressure per square inch of water equals 2.31 feet of water at 60° Fahrenheit.

What is the relation between the pressure and the flow rate in a pump? ›

In general, when pump pressure increases, flow will decrease.

What is PSI and how is it calculated? ›

Pounds per square inch or PSI is an imperial unit of pressure. Using the imperial units of pounds and square inches, it is a measure of force per unit area. Therefore, 1 PSI is measured as one pound of force applied per one square inch.

What is an example of PSI? ›

For example, a bicycle tire pumped up to 65 psig in a local atmospheric pressure at sea level (14.7 psi) will have a pressure of 79.7 psia (14.7 psi + 65 psi). When gauge pressure is referenced to something other than ambient atmospheric pressure, then the units would be pounds per square inch differential (psid).

How do you calculate gpm from hydrant? ›

Converting Hydrant Flow Test Results to GPM - YouTube

### How many GPM is a fire hydrant? ›

Here's the color breakdown a blue topped fire hydrant will have a water pressure of more than 1500 gallons per minute. Green is 1000-1499 gallons per minute, orange is 500-999 gallons per minute, and red is less than 500 gallons per minute.

How many GPM is a hydrant? ›

CLASSHydrants that on individual test usually have a flow capacity of:
CLASS AA1,500 gpm or greater
CLASS A1,000 gpm or greater
CLASS B500 to 1,000 gpm
CLASS C<500 gpm

## Videos

1. Fire hydrant Flow testing, NFPA 291 Math, Trainer version
(John Doe how to)
2. Estimating Flow from a hydrant
(Darren Hall)
3. Chapter 05 Lecture on Water Flow Analysis
(Jim Brakebill)
4. Pump Test: Checking Water Flow with a Pitot Gauge
(Department of Fire Protection Engineering, University of Maryland)
5. hannig ch09 lecture
6. HOW TO: Set Up Accurate Flow Testing
(Elkhart Brass)
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