Short definition
A coupling is the simplest pipe fitting — a sleeve or socket that joins two pipes end-to-end. Variants exist for every joining method: threaded couplings on galvanized and black iron, sweat couplings on copper, solvent-welded on PVC and ABS, compression and push-fit on quick-connects, and stainless-band Fernco couplings on no-hub cast iron and DWV repairs.
What it is
When a plumber says “I’ll add a coupling here,” they mean any of those variants depending on the pipe. The fitting body is short — usually just slightly longer than the pipe diameter — with internal stops or threads matched to the pipe.
A particularly useful subtype is the slip coupling (sometimes called a repair coupling): instead of an internal stop in the middle, the bore is smooth, so the coupling can slide entirely onto one pipe end and then back over the other end after alignment. That makes in-line repair possible without re-cutting and reshaping.
Why it matters to a homeowner
You’ll buy or hear about couplings in three common DIY scenarios:
- Repairing a pinhole leak in copper. Cut out the bad section, install a slip coupling on each end of a new piece of pipe.
- Joining different materials during a partial repipe. A galvanized-to-PEX adapter is essentially a transition coupling.
- Sewer-line repairs. A Fernco coupling joins different-diameter pipes — for example, an old clay-tile section to a new PVC patch — using a flexible elastomer sleeve and stainless-band clamps.
Common variants and what a coupling is not
- Coupling vs. union. A coupling is fixed (cut to remove). A union is a three-piece serviceable joint that unscrews for service without cutting.
- Coupling vs. reducer. A coupling joins same-size pipes; a reducer steps the size down or up.
- Slip coupling vs. standard coupling. Slip couplings have a smooth ID with no internal stop — designed to slide for in-line repair.
- Fernco / no-hub coupling — DWV variant using a flexible elastomer sleeve under stainless-band clamps.