How to calculate water pressure drop

Written by akeia dixon
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How to calculate water pressure drop
The drop in water pressure can be fixed with larger water pipes. (Jupiterimages/ Images)

Calculating the water pressure drop in your home can help you identify the efficiency of certain appliances such as your kitchen sink, washing machine or even the toilet and shower. To fix a problem related to a drop in water pressure, you can replace your toilet with one that is designed for low pressure flushing or replace your water pipes with a pipe that has a larger diameter since these lose less water pressure.

Skill level:


  1. 1

    Check your local building codes to determine the fixture units that tell you the type and size of a water pipe that is recommended to hook up. The standard type of pipes used in most residential construction is 1/2- and 3/4-inch Type M copper pipes.

  2. 2

    Determine the standard flow rate for the particular fixture(s) you are calculating. For instance, the standard flow rate for a toilet is 2.5 gallons per minute (GPM). For a kitchen sink, the GPM is 1.25, 1.6 GPM for a washing machine and 2.5 GPM for a shower. To find the actual flow rate for a fixture, refer to any specifications label on the fixture itself. If the flow rate is not listed, you can run water from the fixture for one minute into a container and determine how many gallons of water were released, or you can refer to a chart, such as that in the Tankless Water Heater Buying Guide, that advises what the typical flow rates are for your specific fixture.

  3. 3

    Refer to a hydraulic friction loss table to determine the constant figure of pressure loss per 100 feet for the amount of GPM you are using. For instance, if you flush the toilet while you are running the shower, the total GPM you are using is 5 GPM. Refer to the chart to determine the figure that represents this number, which is 39.2 for 1/2-inch pipe and 6.94 for a 3/4-inch pipe.

  4. 4

    Multiply this number --- in this case, 39.2 or 6.94 --- by 0.4335, which is a constant that represents the pounds of pressure dropped per square inch. For instance, if you have a 1/2-inch pipe and flushed the toilet while the shower was running, you are experiencing a water pressure drop equal to 16.99 PSI per 100 feet of pipe. So if the pipe is running 200 feet before it gets to the bathroom, multiply this figure by 2 for the accurate pressure drop number, which is 33.98 PSI.

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