Pressure/GPM/Diameter Calculations

  1. I have been searching for a long time and can't figure this out. The formulas I find are way over my head.

    Anyhow, I am trying to find how much flow would go through 2 different sized pipes. One is 3/8" ID and the other is 1/2" ID. The 3/8" uses 26psi and the 1/2" uses 17psi. All I want to know is the GPM each one will flow. I noticed there was a lot about constant of liquids so just use water. As for pressure drop stuff, there is a pump drawing in water and the pipes are on the other side and they have no end on them, so they are just shooting the water out like a hose with no end on it.

    Thanks for any help I can get on this. Sorry if it is a dumb question.
  2. jcsd
  3. Go to

    Click on one of the online calculators on the left-hand side.

    There are also other on-line calculators on the web.

    Bob S
  4. I have gone through that one and many others and I just can't figure them out.
  5. I used the calculator I recommended, and got the following numbers for the pressure drop per 100 ft of pipe:

    1/2" pipe, 2.9 gpm, pressure drop=17 psi per 100' of pipe

    3/8" pipe, 1.3 gpm, pressure drop=24 psi per 100' of pipe.

    If you wold rather not use an online calculator, you can use the Hazen Williams formula at

    Bob S
  6. Ah that works. Think I halfway understand the principles now too. Thanks for the help!
Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook

Have something to add?