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I put in new bathroom vanities that came with the sinks. When I attempted to connect supply lines under the sink I discovered the lines from the faucets are 1/2” female and the supply valve is 3/8” male compression. How can I connect those?

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    Stop posting and go shopping. Until you’ve become a professional plumber, every minor job takes 3 shopping trips: one to start, next for the parts that fit, then the third to replace things that broke. Sep 10, 2022 at 22:40

4 Answers 4

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Hardware stores have a section, usually near the back wall of the plumbing department, with hundreds of small adapter fittings. It can be a minor effort to find the exact part but it's usually in there.

Edit: Home Depot advertised a "3/8 in. Female" adapter but it didn't match the photo.

It sounds like you simply need a reducing union: Some examples:

enter image description here

or

enter image description here

Installed with tubes.

Remove any nuts and ferrules, then screw the 1/2" side into your faucet's 1/2" female supply end.

Then you've got 3/8" male compression on each end and you are ready to add a standard 3/8 comp to 3/8 comp stainless braided water supply hose to complete the connection (you can get short ones, perhaps 6-8" long).

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  • It's important to distinguish between compression threading (which is not tapered) and the more common Iron Pipe Thread (which is tapered). Plenty of 1/2 and 3/8 fittings with FIP (female IPT) or MIP (male IPT), but for faucet water supply connections as here, it's non-tapered threading that's needed, as the watertight seal is not made by the threads.
    – Armand
    Sep 11, 2022 at 18:28
  • @Armand You may be right. I read "3/8 in. Female" in the product listing but that's not how it looks in the photo. 🤔 Normally I just dig through what they have at the store. Sep 12, 2022 at 19:45
  • @Armand I added several more links to help illustrate product availability. Sep 12, 2022 at 20:22
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    Good find on the compression adapters! I proposed an edit replacing the links with the actual photos.
    – Armand
    Sep 13, 2022 at 13:53
  • When I get home I might add one of the images showing the tubes connected to both ends. That was the part I liked on Amazon. Sep 13, 2022 at 14:12
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You either change the faucets, paying attention to the supply connection size this time, or you change the stop valves. Depending on why you changed the sinks out, you might be able to just swap the old faucets over, too.

Or you get into finding adapters, recognizing that each extra connection for those adds another place to potentially leak.

Having a fit doesn't make it go any faster.

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    Just a note that this is a relatively new problem, as in the 2010s manufacturers started including non-removable water supply lines in their faucets. This saves manufacturing cost and simplifies installation. In the "old days" of mix & match water supply lines, one would just get a line with the appropriately-sized connector on each end for one's shutoff and for the faucet.
    – Armand
    Sep 11, 2022 at 18:33
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If the pre-installed lines in your vanity are of low quality and can be removed, remove them, and hopefully you can replace them with quality braided steel lines that suit the faucet at one end and the valve at the other without adapters.

If not just use a 3/8 female to 1/2 male adapter like this, that you'll find at any hardware store. (Note that with the one in the picture you'd use tape on the valve side, but rely on the line's rubber washer on the line side).

enter image description here

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  • The 3/8" shutoff male end will have non-tapered threads. Probably not a good idea to try connecting a tapered-thread adapter (as I assume by your tape reference) to that in a situation where it will be under constant water pressure.
    – Armand
    Sep 14, 2022 at 15:47
  • @Armand I know tapered threads require PTFE, but I've never heard that PTFE tape is wrong or harmful for straight threads. It certainly works. Also, OP may find an adapter that uses a gasket in the 3/8 female end and requires no other sealant. I'll take your point though .... OP should seal both sides of the adapter with the correct appropriate materials, which depend on exactly what he buys.
    – jay613
    Sep 15, 2022 at 14:42
  • As you know, non-tapered threaded connections seal by either a rubber washer at the end or by a specialized metal to metal clamping action (compression ferrule). Either is likely to leak if teflon tape gets between the sealing surfaces. Like you say, tape is not inherently a problem but IMHO especially in DIY practice likely to sometimes cause leaks because of that issue. At least with plumbing projects, leaks should be evident right away. :)
    – Armand
    Sep 15, 2022 at 17:48
  • @Armand since you seem to have relevant knowledge may I call my question to your attention, particularly the part that asks how to seal a certain threaded connection?
    – jay613
    Sep 15, 2022 at 18:23
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It is very very easy: You go buy a 1/2" female to 3/8ths inch male braided hose. This one is a ' B1F16 3/8" x 1/2" x 16" Flex Faucet Supply Stainless Steel', and I'm sure that if you search for it online or at your local big box store, you'll be able to find it.

It installs very quickly. You might also check if toilet connectors work on your plumbing.

braided hose with different ends

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  • I think OP's vanity has pre-installed lines, not 1/2" connectors on the faucets.
    – jay613
    Sep 13, 2022 at 18:47
  • Yes, the OP has a 1/2" female end on their faucet line, not a male end.
    – Armand
    Sep 14, 2022 at 15:43
  • In that case you buy the 1/2 1/2 male union and the gadget above. problem solved. Or you get a female 3/8th rubber to 1/2 male adapter. But you can get cheap and easy off the shelf parts for this that just screw together.
    – gbronner
    Sep 14, 2022 at 21:08

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