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Community and Q&A

3/4 pex max flow rate

ChrisInIllinois | Posted in GBA Pro Help on

The short of it:

I can’t seem to find a chart on max gpm that it can handle. I know it depends on max safe flow rate, in coming psi, and pressure loss.

I’m going to pex manifold, but will have a whole house water system in the line that is rated for 14 gpm max.

The long details:
City Home is 3/4 pvc schedule 40 in from  main, then it’s reduce 1/2 ,trunk and branch copper all done with 1/2. 

I’m trying to to see if I need to come to water filter with 3/4 cpvc ,copper  or if 3/4 pex could handle the supply load.

Maximum load would be filling bath, while filling washer, while filling dish washer, while brushing teeth……. It happens to often things have to be paused. 1 bath 4 person house.

Thanks for your time.

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  1. Expert Member
    BILL WICHERS | | #1

    A chart of flow rate:

    A chart of pressure drop:

    A nice guidebook with some good flow rate charts:

    Note that you need to allow for the length of the run when you’re determining flow rate. It’s similar to volt drop in electrical wiring. On long runs, you need to upsize the pipe to get the same flow rate as a smaller pipe on a shorter run.


    1. ChrisInIllinois | | #3

      That's quite funny, I have read all them, + more from others. Seems 3/4 will work in my setup less water filter based on a HUD book.

      They flow charts from supply house are based on max 4ft/sec (think it's for heating). uponor rates there supply chart up to 12ft/ sec.

      The pressure drop chart from pex universe has up to 12 gpm with a psi drop of around 25 based on 100 foot.

      Technically then 50 foot should see a drop of around 12 psi?

      I get all this is based on manufacturer and there specs and connection system used. I just can't in good faith take the easy road 3/4 pvc to 3/4 pex with out knowing the numbers. It's more work and costlier to 3/4 pvc to 3/4 cpvc/copper but if that's ran to the water filter then split to two 3/4 pex I know the numbers work.

      Anyone recommended push fits on the water main entrance before in house shut off or stick to old school solvent welding threaded adapter?

      Thanks, seems this may have branches of topic a bit. Sorry.

      1. Expert Member
        BILL WICHERS | | #6

        Higher flow velocity means more pressure drop per unit distance, and also more wear on parts (that’s a design issue in big systems I work with at work).

        In your system, I’d just go by relative inside diameters. PEX has a little less friction, so a little better flow for the same size as PVC, but the difference is too small to be noticeable in your application. PEX also has less flow restriction when you form sweeping bends instead of using 90 degree fittings. If you’re using 3/4” PVC now, 3/4” PEX isn't going to make any real difference in your flow rates.

        There is nothing to be gained by transitioning to 1” from a 3/4” supply if it’s only for a short distance. If you have a flow issue with the filter itself, a 1” filter might offer less backpressure, but that would be based more on the construction of filter (more filter media area maybe) than on the size of the pipe fitting. Using larger pipe after a 3/4” supply will only help if you are seeing reduced flow at the end of a long run of pipe and you’re trying to replace that long run with the larger pipe.


        1. Deleted | | #7


          1. Expert Member
            BILL WICHERS | | #11

            It sounds like your water supply from the street is 3/4”. Just connect it to your filter and be done with it. The 3/4” LINE is your limit here, nothing you can do short of adding a pump is going to increase your max incoming flow rate, period. Whether or not you use the filter is irrelevant. You won’t get any more flow than that 3/4” supply line can provide.


  2. tommay | | #2

    Q = VA and Bernoulli

    1. ChrisInIllinois | | #4

      So I don't know what governs q=va or if it's total not counting friction loss.

      But at o.681 inch (a= pi*D*D/4) 3/4 pex. And safe velocity of 12 ft / sec.

      Using a online calculator to do all the conversions. That's around 13.6 gpm, before losses, so 8 to 10 gpm max after losses.

      Does that seem about right? Then I should step it up to tired 3/4.

      1. tommay | | #5

        nothing you can do, if your incoming line is 3/4, hook that up to your filter. If you change it, the volume and pressure will change accordingly.
        if you only want to filter certain lines, a little plumbing may have to be done.

        Oh by the way Q, volume flow rate = velocity x area
        you can look up bernoulli's

  3. vivian_girard | | #8

    Unless you do some highly unusual things or have really low water pressure, a 3/4" water line of pex or anything else should be way more than enough for a family of 4. I have zero calculation to back this up, but I have some experience; I own and manage a small apartment building with 7 bathrooms, 4 kitchens and 12 adults living in it full time. The building has a 5/8" water meter, and 3/4" pex for the main line after that. We also have low flow shower heads everywhere. Water pressure is never an issue.

  4. walta100 | | #9

    I say that the safe bet is no PEX before the manifold.

    You need to be careful when you say PEX there is more than one type of PEX and several different fitting systems. In the most common systems the fitting go inside the pipes and restrict the flow much more than the pipes. In the lowest cost system the 3/4 inch fittings have an inside diameter just slightly over 1/2 inch and the 90° fitting have 0 radius very restrictive.

    If you are planning a PEX home runs to a manifold system you may want to reconsider your choice.
    It requires twice as much materials and twice as much labor and if you are using hot water from 2 taps the total wait time and wasted water will often be greater.


  5. seabornman | | #10

    I second Walter's comment re: waiting for hot water. I thought I'd be smart and run 3/4" to my kitchen which is 45 ft. from WH. Now I have to install a recirc system or similar due to the wait for hot water.

  6. ChrisInIllinois | | #12

    I want to thank everyone for there time and replies. To anyone reading this in the future for help, outside of the links bill (zephyr7) posted above I also find this to be helpful too:
    Testing done by NAHB

    HUD design guide
    Design Guide - Residential PEX Water Supply ... - HUD User › portal › publications › pex_design_guide

    And look on your pex manufacturer web page read there documents.

    I think I'm going to just go to 3/4 pex. I will report back my results and if I had to change things. (This may take a bit I'm still running an old water heater with 1/2 partially plug lines. I don't want to move or touch until I have a replacement, it's from 1984 not a typo that's 34 years. Still working with a bit of rust.)

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