Many home plumbing projects mix old and new: an existing copper system with a new run of PEX for a bathroom remodel, a basement finish or a repair. Connecting PEX to copper is straightforward once you pick the right transition fitting. There are three common methods: sweat (solder) adapters, push-to-connect fittings, and crimp or expansion adapters that join to the copper by solder, press or thread.
This guide explains how each method works, when to use it, and the mistakes to avoid.
Before you start: know your pipe sizes
Most residential copper and PEX in the US uses copper tube size (CTS), so 1/2" PEX matches 1/2" copper tube and 3/4" matches 3/4". Even so, confirm what you have:
- Copper: Type M, L or K copper all share the same outside diameter for a given nominal size. A 1/2" copper tube measures about 5/8" outside diameter, and 3/4" measures about 7/8".
- PEX: Check the print line on the tubing for its size, type (PEX-A, B or C), and the fitting standards it's listed for, such as ASTM F1807, F2159 or F1960.
Not sure which PEX you have? Our guide to PEX-A vs PEX-B explains the differences and the matching fitting systems.
Safety and preparation
- Shut off the water at the main valve or the nearest fixture valve, open a faucet below the work area to drain the line, and keep a bucket and towels handy.
- Soldering uses an open flame. Wear safety glasses, keep a fire extinguisher nearby, protect nearby framing with a heat shield, and check the area for smoldering after you finish. Use lead-free solder and flux rated for potable water.
- Electrical bonding: In many homes, metal water piping is part of the electrical grounding or bonding system. Replacing a section of copper with PEX can interrupt that path. If you cut into a copper line, especially near the water meter or main shut-off, have an electrician check whether a bonding jumper is needed.
- Water heaters: Many PEX manufacturers and local codes don't allow PEX to connect directly to a gas water heater. Use metal pipe or approved connectors for a minimum distance (often 18 inches) from the heater, and follow the tubing manufacturer's instructions.
Method 1: Sweat (solder) adapters
A sweat adapter has a copper socket or spigot on one end and a PEX barb on the other. You solder the copper end onto the existing copper pipe, then attach the PEX with a crimp, clamp or expansion connection. Common versions include female sweat x PEX and male sweat (fitting) x PEX adapters.
How to do it
- Cut and prepare the copper. Cut the copper square with a tubing cutter. Deburr the inside and outside, then clean the pipe end and the fitting socket with emery cloth or a fitting brush until shiny.
- Solder the adapter before attaching the PEX. Apply flux, assemble, heat the fitting evenly, and feed solder into the joint. Let the joint cool completely. Never heat a fitting with PEX already attached, because the flame or conducted heat will damage the tubing.
- Attach the PEX. Once the adapter is cool, cut the PEX square, slide on a crimp or clamp ring, push the PEX fully onto the barb, and make the connection with the correct tool. Check crimp rings with a go/no-go gauge.
- Test. Turn the water back on slowly and check every joint for leaks.
Tip: If you can, solder the adapter onto a short piece of copper first (on a workbench), then connect that assembly to the existing pipe with a coupling. This keeps flame work away from walls and lets the joint cool before you work near the PEX.
Best for: Permanent, concealed connections where you're comfortable soldering. Sweat joints are proven and inexpensive. Find fittings and couplings in our copper sweat fittings collection.
Method 2: Push-to-connect fittings
Push-to-connect fittings (such as SharkBite and similar brands) join copper, PEX and CPVC without solder or crimp tools. Inside the fitting, a stainless steel grip ring holds the pipe, and an O-ring seals it. A push-to-connect coupling can join 1/2" copper directly to 1/2" PEX.
How to do it
- Cut both pipes square with a tubing cutter (copper) and a PEX cutter (PEX).
- Deburr and clean the copper. A sharp edge or burr can cut the O-ring. Remove any burrs and wipe the pipe clean. The pipe surface must be smooth and free of scratches or flattening where the O-ring seals.
- Check for a PEX stiffener. PEX needs a tube support (stiffener) inside the pipe end. Many brass push fittings include one already installed. If yours doesn't, insert one according to the fitting instructions.
- Mark the insertion depth on each pipe, using the depth gauge or the fitting's instructions.
- Push each pipe in firmly until it reaches the mark. Turn the water on and check for leaks.
Best for: Quick repairs, tight spaces where a torch isn't safe, and anyone without soldering or crimping tools. Many push fittings can be removed with a release clip or tool and reused, depending on the manufacturer. Check the fitting's listing and local code before burying one in a wall or using it underground. Shop push fittings.
Method 3: Crimp or expansion adapters (with threaded or press options)
If you already have PEX tools, you can use a transition adapter made for your PEX system and connect it to the copper side in whatever way suits the job.
Crimp and clamp adapters (F1807 / F2159)
These have a PEX insert barb for crimp or clamp rings on one end and a copper connection on the other: a sweat end, a male or female thread, or a press end. They work with PEX-B, PEX-C, and PEX-A tubing that's listed for crimp or clamp connections. Browse PEX crimp fittings (F1807) for sweat and threaded adapters.
Expansion adapters (F1960)
For PEX-A expansion systems, adapters come with an expansion fitting on the PEX end and a copper sweat or thread on the other. Make the copper side first (solder and cool, if sweating), then expand the PEX-A tubing and ring and push it onto the adapter. Allow the joint to shrink fully before pressurizing it.
Threaded transitions
A threaded transition avoids soldering near the PEX: solder a female or male threaded adapter onto the copper, then thread on a matching PEX adapter. Use thread sealant tape or pipe dope rated for potable water, and don't overtighten. Threaded joints also make it easy to disconnect later.
Press transitions
If you have a copper press tool, you can press a fitting onto the copper and use a press x PEX adapter, which avoids an open flame entirely. Use fittings and jaws from the same press system.
You'll need the right cutter and crimp, clamp or expansion tool for the PEX side. See our PEX tools.
Which method should you use?
| Method | Tools needed | Pros | Watch out for |
|---|---|---|---|
| Sweat adapter | Torch, solder, flux, plus PEX crimp/clamp/expansion tool | Permanent, proven, low fitting cost | Open flame; heat damage to PEX if soldered with tubing attached |
| Push-to-connect | Cutters and deburr tool | Fast, no flame, often removable | Higher fitting cost; pipe must be clean and undamaged; check code for concealed use |
| Crimp/expansion adapter (sweat, thread or press) | PEX tool, plus torch, wrenches or press tool | Matches your existing PEX system; flexible options | Use rings and fittings listed for your PEX type |
Common mistakes to avoid
- Soldering with PEX attached. Always solder first and let the joint cool.
- Skipping deburring. Burrs on copper can cut O-rings in push fittings and cause leaks.
- Mixing fitting systems. Don't use F1960 expansion fittings on PEX-B, or crimp rings from one system with fittings that aren't listed for them.
- Not supporting the transition. Secure both the copper and PEX near the fitting so the joint doesn't carry the weight or movement of the pipe.
- Forgetting the bonding jumper when replacing copper that's part of the electrical bonding path.
If you're unsure about soldering, gas water heater connections or electrical bonding, hire a licensed plumber. For help matching adapters to your pipe, send us your pipe size and PEX type and we'll help you find the right fittings.