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How to Calculate Water Flow in a Hose by Using Pressure

By Pauline Gill ; Updated September 21, 2017
A wide-open garden hose has the greatest flow.

Hoses provide the ultimate in utility and flexibility in moving a liquid or gas from one place to another safely and efficiently. A wide assortment of hoses are available for domestic and gardening use, industrial applications and in fighting fires. In every case, hose-system designers seek to provide maximum flow with minimum pressure losses, and they use Bernoulli’s Principle, which simply states that flow through a tube is proportional to the square root of pressure change through a system. You can calculate flow from pressure information using published hose data.

Garden Hose

Flow always finds the path of least resistance.

Define the flow application. In this example, a 50-foot garden hose with a 5/8-inch ID (0.625-inch inner diameter) is being used to fill a pressurized tank with 5 psi of fixed pressure. The water pressure at the hose connection to a well pump is 45 psi. From this information you can calculate water flow through the hose using pressure.

Nozzles restrict pressure but increase velocity.

Calculate the total pressure drop or differential through the 50 feet of hose. Subtract the outlet pressure, 5 psi, from the inlet pressure, 45 psi, to yield the 40 psi total loss through the hose.

Look up the flow rate for 40 psi of head loss through 50 feet of 5/8-inch ID hose. In this case, data for Schedule 40-PVC plastic pipe of the same inner diameter may be used to determine the flow. Schedule 40-PVC plastic pipe has an inner diameter of 0.622 inches. The pressure-loss data for this pipe shows 35.5 psi loss for 10 gallons per minute (gpm) through 100 feet of pipe. To compute the flow for 40 psi through 50 feet of hose, take the square root of (40 psi loss/[35.5/100 feet/50 feet]) and multiply by 10 gpm to yield 15.01 gpm.

Fire Hose

Fire hoses are much larger than garden hoses.

Define the fire hose application. A 100-foot, 3-inch-diameter fire hose is being tested for use to drain a large swimming pool using a pump that delivers 55 psi. You can calculate the flow through the fire hose with this information.

Pressure quickly opens this flattened fire hose to completely round.

Look up the specific flow vs. pressure data for the fire hose. The manufacturer’s pressure loss data shows that a 3-inch ID hose loses 57.8 psi for an 850 gpm flow.

Calculate the exact flow rate by correcting the data to actual pressure loss. Take the square root of 55 psi/57.8 psi, which is 0.9755, and multiply by 850 to get 829.16 gpm.

 

Things You Will Need

  • Calculator
  • Published hose or thermoplastic pipe flow-pressure data

Tip

  • Always multiply your flow needs by 1.25 when determining the right hose size for the job in order to add greatly to efficiency.

Warning

  • The propulsive forces of fire hose nozzles can quickly do damage and cause serious injury if not handled correctly.

About the Author

 

Pauline Gill is a retired teacher with more than 25 years of experience teaching English to high school students. She holds a bachelor's degree in language arts and a Master of Education degree. Gill is also an award-winning fiction author.