How to Calculate Water Flow Through a Nozzle

Written by katherine ragland
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How to Calculate Water Flow Through a Nozzle
Nozzle size affects water flow. (water spout image by tomcat2170 from

Intuitively, we know that when we decrease the size of the water hose outlet, the stream of water flows out faster. This simple principle applies to all water flowing out of a nozzle. When the nozzle diameter decreases, the water flow velocity increases. Thus, when we want to softly spray our flower beds with a gentle shower, we increase the diameter of our nozzle. To get a more forceful stream, with perhaps enough velocity to clean off our driveway, we decrease the nozzle diameter.

Skill level:

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Things you need

  • Measuring tape

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  1. 1

    Measure the diameter of your water hose. Divide this by 2 to get the radius. Make a note of this measurement.

  2. 2

    Measure the diameter of your nozzle outlet. Divide this by 2 to get the radius. Make a note of this measurement.

  3. 3

    Measure how much water flows out of your nozzle in one minute. Take a cup or large pot, depending on the amount of water flowing from your nozzle, and let the water flow for 10 seconds. Then, measure the volume of water collected. The velocity of the water through your hose equals this measurement over 10 seconds.

  4. 4

    Calculate the velocity of the water exiting the nozzle using this equation: Exit velocity = Velocity of water through hose (measurement from step 3) X ( [(Radius of water hose (measurement from step 1)^2) / (Radius of nozzle outlet (measurement from step 2)^2)]. For example, for water flowing at 1 L/10 seconds, with a hose radius of 20 cm and a nozzle radius of 5 cm, the exit velocity would equal ((1 L/10s)*[(20 cm^2)/(5 cm^2)]), or 1.6 L/second.

Tips and warnings

  • Use a large measuring cup to simplify Step 3.

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