How to connect a faucet and pipe with different sizes
Supply lines and shut-off valves frequently come in mismatched dimensions, such as 3/8-inch and 1/2-inch fittings, or P-traps that don't fit standard sink drains. This dimensional conflict prevents DIYers from completing basic plumbing installations without buying specialized adapters or alternative components.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
- 1Buying a plumbing size adapter from a big box store
- 2Installing a straight stop valve that adapts the connection size
- 3Returning and replacing the faucet with one matching the existing connections
- 4Using reducing bevel washers to adapt smaller tailpieces to larger traps
- 5Using couplings and bushings to step down pipe sizes from the wall
- 6Checked multiple local hardware stores for matching threads or custom threading services.
- 7Buying adapters or different styles of tub spouts
- 8Unsoldering or uncoupling the existing pipe fitting
- 9Buying a differently sized adapter
- 10Replacing the entire integrated valve with a standard angle stop
- 11Disconnecting the white hose and attempting to connect directly at the valve
- 12Remove the incorrect valve or angle stops and connect the 1/2 CPVC directly to the valve using proper solvent connections or ball valves.
In their words
Unedited, most upvoted first, each linked to the thread it came from.
“Based on brief research it seems I may have a 3/8 and 1/2 inch mismatch, do I just buy an adapter or if the faucet is refundable should I get a different one.”source ↗
“I believe the p trap I have is too big for the sink drain. What would be the best way to connect this?”source ↗
“Broke first time home owner trying to do what I can but coming up short. Any help is very appreciated.”source ↗
“Basically, to summarize, the fittings in Singapore have the same sizes but the threads dont match therefore i cannot screw into the water system.”source ↗
“Ive checked about 8 stores, none have a matching thread nor can custom.”source ↗
“Not sure what to do here to connect the below system to the SG HDB water inlet and out let”source ↗
“I’m replacing my bathtub spout and want to switch to a matte/flat black tub spout with a diverter.”source ↗
“I originally bought a slip-on/set-screw style spout, but there’s only about 3/4" of plain copper before the soldered fitting, so I don’t think that style will work without redoing the plumbing.”source ↗
“I’d really prefer to leave the existing pipe/fitting alone and buy a spout or adapter that will work with what’s already there.”source ↗
“Trying to avoid buying the wrong style again.”source ↗
“I wasn’t sure how to do that with the white hose connected like this:”source ↗
“Alright so I didn't realize that the valves that are installed have 3/8 inch threaded outputs. Can anyone tell me the best way to make this valve work with 1/2 CPVC lines?”source ↗
“I intended on just using 1/2 shark bite connectors but didn't realize it was 3/8.”source ↗
“How would you go about connecting the supply line from the bathroom faucet (black) which is 3/8 to the white flexible pipe line coming from the water supply.”source ↗
“I thought the water supply line was 3/4 but I bought this 3/8 to 3/4 adapter and the water line didn’t fit.”source ↗
Where this came up
People with this problem also raised
- 4How to connect a sink drain to existing glued PVC pipes
- 10How to figure out confusing plumbing and water flow directions
- 9How to fix an off-center toilet drain during a remodel
- 49How to fix a leaking water pipe in a tight space
- 12Why does my new plumbing still have bad water pressure and risky joints?
- 2How to deal with failing and rusted galvanized pipes