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Estimates for planning. Use the pipe inside diameter, not the outside diameter, for accurate results.
GPM stands for gallons per minute, the most common way to express water flow rate in the United States. It is the volume of water that passes a point in one minute. A garden hose delivers a modest flow, while a fire hydrant delivers far more, but both are measured the same way, in gallons per minute.
Flow rate is different from pressure. Pressure is the force pushing the water, measured in psi. Flow rate is how much water actually moves, measured in GPM. A pipe can have high pressure and low flow, or the reverse, so the two are measured separately.
Flow rate is one of the numbers a utility checks constantly. It determines whether a service line can supply a building, whether a hydrant meets fire-flow requirements, and whether a pump is performing to its curve. It is also the basis for estimating usage and spotting leaks when metered flow does not match expected demand.
Because flow readings tie directly to meters, service lines, and billing, they belong with the rest of a utility's operational data. A connected water utility management software platform keeps meter, consumption, and asset records together, so a flow figure can be checked against the account and the pipe it came from.
The calculator offers two methods. The pipe method uses the pipe's inside diameter and the water velocity. The volume method uses a measured volume and the time it took to collect, which is the classic bucket test.
The constant 2.448 in the pipe method already accounts for converting inches and feet per second into gallons per minute, so you only need the diameter and velocity.
Two examples, one for each method:
Not sure which method to use?
Use the pipe method when you know the pipe size and velocity, and the volume method when you can physically time filling a container:
Three mistakes are common. The first is using the pipe's outside or nominal diameter instead of the inside diameter, which changes the area and throws off the result. The second is confusing pressure with flow, since a high-pressure line does not always carry a high flow. The third is mixing up time units in the volume method, so remember the calculator expects seconds, not minutes.
There are two common ways. From pipe data, multiply 2.448 by the square of the inside diameter in inches and by the velocity in feet per second. From a fill test, divide the gallons collected by the seconds it took and multiply by 60.
Yes. Use the volume method, also called a bucket test. Collect water in a container of known size, time how long it takes to fill in seconds, then divide the gallons by the seconds and multiply by 60 to get gallons per minute.
Divide the flow rate in gallons per minute by 448.8. One cubic foot per second equals about 448.8 gallons per minute. The calculator shows this conversion automatically.
These related calculators cover the rest of a water and wastewater system:
To see how one platform keeps meter, consumption, and asset records behind figures like these in a single place, explore SMART360 water utility management software.
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