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How to Calculate Water Flow for a Given Pipe Size

Water flow is a calculation used for builders when determining the overall fixtures and plumbing for a building. If the building is in an area with lower water flow from the municipal pipes, then use this equation for determining the necessary pipe size for optimal water pressure.

Picking the right pipe for the job ensures a longer life for the building's plumbing.

Things You Will Need

  • Calculator with pi function
  • Velocity-type water meter

Water flow is a calculation used for builders when determining the overall fixtures and plumbing for a building.  If the building is in an area with lower water flow from the municipal pipes, then use this equation for determining the necessary pipe size for optimal water pressure.

    The symbol for pi. Not all calculators are equipped with such a key.
  1. Multiply pi by 0.25.
  2. Press the multiply button again and enter the pipe diameter squared.
  3. Press the multiply button again and enter the velocity of the entering water. The velocity of water is obtained by attaching a velocity-type water meter to the pipe and running the water. Lengths of time necessary for a velocity reading vary depending upon the manufacturer, so review your meter's user manual for testing requirements.
  4. Press enter. The answer is your water's flow rate.

Things You Will Need

  • Calculator with pi function
  • Velocity-type water meter

About the Author

Steven White is a privately contracted software engineer and efficiency analyst. He has more than five years of experience providing technical support for AT&T broadband customers. Along with his technology background, White enjoys carpentry and plumbing.

Photo Credits

  • water pipes image by Byron Moore from Fotolia.com
  • water pipes image by Byron Moore from Fotolia.com
  • pi image by Ewe Degiampietro from Fotolia.com