A plumbing fitting is any connector piece used to join sections of pipe together, change the direction of flow, branch a line, or transition between different pipe sizes. Fittings are what transform individual lengths of pipe into a functional plumbing system, making it possible to route water supply lines through walls, connect drains under sinks, and build networks far more complex than any single pipe could handle.
How Fittings Differ From Pipes
Pipes are the straight runs that carry water or waste from point A to point B. Fittings are the connecting hardware at every turn, junction, and endpoint. Without fittings, you’d have nothing but disconnected tubes. Every time a water line needs to turn a corner, split into two directions, change diameter, or terminate at a fixture, a fitting makes that happen.
Fittings also serve as transition points between different pipe materials or connection types. If your home has older copper supply lines but a repair uses modern PEX tubing, a special adapter fitting bridges the two materials into a single, leak-free connection.
Common Types of Fittings
Most residential and commercial plumbing systems rely on the same handful of fitting types, each designed for a specific job.
- Elbow: Changes the direction of a pipe run. The most common angles are 90 degrees (a sharp turn) and 45 degrees (a gentler bend). You’ll find elbows everywhere, from the turn under your kitchen sink to the bends inside your walls.
- Tee: Shaped like the letter T, this fitting connects three pipes at a single junction. Tees let you branch a main water line to feed multiple fixtures, like splitting one supply line to serve both a bathroom sink and a shower.
- Coupling: A straight connector that joins two pipes end to end in a permanent connection. Couplings are used to extend a run when a single pipe isn’t long enough or to repair a damaged section by splicing in new pipe.
- Union: Similar to a coupling but designed to be taken apart later. Unions have a threaded nut in the middle that lets you disconnect pipes for maintenance or replacement without cutting anything.
- Adapter: Converts one connection type to another, such as switching from a threaded connection to a soldered one, or stepping down from a larger pipe diameter to a smaller one.
- Cap and plug: Seal off the end of a pipe. Caps fit over the outside of a pipe opening, while plugs fit inside it. Both are used to terminate a line that may be extended later or to close off an unused branch.
- Reducer: Transitions between two different pipe sizes within the same run. A reducer lets a 3/4-inch supply line step down to a 1/2-inch line feeding an individual fixture, for example.
Fitting Materials and Where They’re Used
Fittings are made from the same materials as the pipes they connect, and the right choice depends on the application. Copper fittings pair with copper pipe and are soldered together, a method that’s been standard in water supply lines for decades. Brass fittings are common for threaded connections and valves because brass resists corrosion and holds threads well over time.
PVC and ABS plastic fittings dominate drain, waste, and vent systems. They’re lightweight, inexpensive, and joined with solvent cement (a chemical adhesive that fuses the plastic). You’ll recognize these as the white or black pipes and fittings under sinks and running through basements.
PEX systems, increasingly popular for water supply lines, use their own category of fittings. These are typically brass or a corrosion-resistant engineered polymer called polyalloy. Polyalloy fittings are a practical alternative in areas where water chemistry is aggressive enough to corrode brass over time. PEX fittings connect using crimp rings, clamp rings, or expansion connections rather than solder or glue, making them faster to install.
For drinking water applications specifically, fittings must be rated for potable water use. This matters because fittings intended for irrigation, hydroponics, or sewage systems may not meet the same safety standards for lead content and material purity.
How Fitting Sizes Work
Plumbing fittings in North America follow the Nominal Pipe Size (NPS) system, and the naming convention can be confusing at first. The “size” of a fitting doesn’t refer to an exact measurement you could verify with a tape measure. For pipes and fittings sized 1/8 inch through 12 inches, the nominal size and the actual outer diameter are different numbers. An NPS 12 pipe, for instance, has an actual outer diameter of 12.75 inches.
This quirk exists because the system was originally based on inside diameter, calculated using wall thicknesses that were standard at the time. As manufacturing evolved, the names stuck even though the math no longer lines up neatly. For pipes 14 inches and larger, the nominal size does equal the actual outer diameter, so the confusion mostly affects the smaller sizes used in residential work.
The practical takeaway: always match fittings to pipe by their labeled nominal size, not by measuring with a ruler. A 1/2-inch fitting connects to 1/2-inch pipe, even though neither dimension is literally half an inch.
Threaded vs. Non-Threaded Connections
Fittings connect to pipes in several ways. Threaded fittings screw onto pipe ends and are common on steel and brass pipe. Non-threaded methods include soldering (copper), solvent cementing (PVC and ABS), crimp or expansion connections (PEX), and compression fittings that tighten with a nut and ferrule.
Within threaded fittings, there’s an important distinction between tapered threads and straight threads. Tapered threads (designated NPT, for National Pipe Taper) get progressively tighter as you screw them in, which is what creates the seal. These connections still need thread sealant tape or pipe compound to be fully leak-proof, because the seal depends on the threads wedging tightly together.
Straight threads (designated NPS, for National Pipe Straight) are parallel rather than tapered. They can’t form a seal on their own, so they rely on a gasket or O-ring compressed between the fitting faces. Tapered and straight threads will physically engage with each other since they share the same thread angle and pitch, but they won’t seal properly together. Mixing the two types is a common source of mysterious leaks.
Choosing the Right Fitting
Selecting a fitting comes down to four questions: what material is your existing pipe, what size is it, what do you need the fitting to do (turn, branch, join, reduce), and what’s the connection method? Match all four, and the fitting will work. Get any one wrong, and you’ll end up with a leak or a trip back to the hardware store.
For water supply repairs, the pipe material usually dictates your options. Copper pipe gets copper or brass fittings. PEX gets PEX-compatible fittings in brass or polyalloy. For drain lines, PVC fittings matched to the correct PVC schedule (a wall-thickness rating) are standard. If you’re transitioning between materials, adapter fittings exist for nearly every combination, including copper-to-PEX, PVC-to-cast-iron, and threaded-to-unthreaded conversions.