I am trying to connect an under counter Reverse Osmosis ( R.O. ) filter inlet line (red 1/4" tubing) to the cold water supply line (3/8" black tubing in picture) under my kitchen sink.

Can anyone help me figure out what sort of Tee/adapter fitting I need to get this done?

Obviously I still need the black cold water supply line to feed the sink faucet as well as this red R.O. Filter inlet line.

I also want the shut-off valve to be in the line to the R.O. filter.


enter image description here

RO inlet

Additional pictures here showing sizes of connections: https://i.sstatic.net/nuXhV.jpg https://i.sstatic.net/ze4gc.jpg https://i.sstatic.net/F7nlz.jpg

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    Commented Aug 27, 2020 at 23:54

1 Answer 1


You need a tee that has two females and one male. The typical size for this situation is 3/8" or 1/2", you note that you have 3/8"

Use Teflon tape on the threads too.

Note that one of the images below is 2M1F but is otherwise visually representative of what you need

enter image description here enter image description here

  • 1
    Don't use teflon tape on compression fittings or fittings that have an O-ring. Those seal in other ways. Most connectors of this type will be one of those 2, so most likely no need for teflon tape in this case. Commented Aug 27, 2020 at 18:38

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