Pipe fittings are the connectors that hold a plumbing system together. They join one pipe to another to extend a run, change the direction of flow, reduce the line to a smaller size, or split the flow into two branches. Builders also call them pipe connectors, and each fitting follows the size of the pipe it serves. A fitting with two same-size openings acts as a transition between equal pipes; different openings step the line to a new diameter. The full pipe fittings overview at build-construct.com covers this component family in more detail.
Sizing standards matter because a fitting must match the nominal size, wall thickness, and joining method of the pipe. Some fittings change direction; others change size; a few do both. The sections below cover each major type, its job, and where it belongs.
What Pipe Fittings Do and How Sizing Works
A fitting solves one of four problems: extend the run, change direction, reduce or enlarge the line, or combine and divert flow. Choosing starts with the pipe material, the fluid pressure and temperature, and the joint method used elsewhere in the system. On plastic lines the joint method shapes the fitting selection, and the PVC pipe connection methods guide shows how solvent-weld, threaded, and push-fit joints each call for a different fitting style.
The Job of a Pipe Fitting
- Extend a run: couplings and unions join two lengths of pipe end to end.
- Change direction: elbows and bends turn the flow at 90, 60, 45, or 22.5 degrees.
- Reduce or enlarge: reducers and bushings step the line to a new diameter.
- Combine or divert: tees, crosses, and olets split one flow into two or more branches.
- Stop flow: caps and plugs close a line for testing or permanent termination.
- Allow maintenance: unions and flanges let a section be removed without cutting the pipe.
Sizing Standards and Dimensions
Fitting dimensions follow pipe dimensions. Nominal pipe size (NPS) names the approximate inside diameter, schedule describes wall thickness, and both must match. A fitting rated for the same NPS and schedule keeps the bore consistent and avoids turbulence at the joint. Pressure ratings follow material and wall thickness, so a fitting never carries a rating lower than the pipe it connects.
- Measure the outside diameter of the pipe and identify the material.
- Match the nominal pipe size and schedule to the fitting.
- Compare the pressure and temperature rating of the fitting with the system.
- Confirm the joint type matches the joining method: thread, solder, solvent weld, or push-fit.
| Fitting | Primary job | Typical materials |
|---|---|---|
| Elbow | Change flow direction | Brass, copper, PVC, steel |
| Tee | Split flow into a branch | Copper, CPVC, PEX, steel |
| Reducer | Step the line to a smaller size | Brass, PVC, stainless steel |
| Coupling | Join two pipes end to end | Copper, PVC, steel |
| Union | Join pipes with easy disassembly | Brass, galvanized steel |
| Cap or plug | Close the end of a line | PVC, brass, steel |
| Olet | Weld a branch onto a main | Carbon steel, stainless steel |
Direction-Changing Fittings: Elbows and Tees
Elbows are the most common fittings in a piping system because runs rarely stay straight. Standard bend angles are 90, 60, 45, and 22.5 degrees, and elbows can be joined in sequence to move a line around obstructions. The types of pipe fittings used in plumbing at civillead.com lists the same family with extra sizing detail.
90 Degree Elbows
The 90 degree elbow is the standard turn fitting, and two radius classes matter in tight layouts.
Long Radius vs Short Radius
A long radius elbow has a centerline radius of 1.5 times the nominal pipe size, so a 2 inch fitting curves around a 3 inch radius. The gentler turn lowers pressure drop and turbulence, making it the default for water and process lines. A short radius elbow uses a radius equal to the pipe size, saving space but adding friction loss. Use short radius elbows only where the layout forces a compact turn and pressure drop is acceptable.
45 Degree Elbows
A 45 degree elbow turns the flow at half the angle of a standard bend. It suits gradual direction changes, offsets between parallel pipes, and drainage lines where a sweeping turn keeps solids moving. Two 45s joined together make a 90 degree turn with less turbulence than a single short radius elbow.
Tees: Equal and Reducing
A tee joins three pipes: the run passes straight through and the branch leaves at 90 degrees. An equal tee has all three openings the same size. A reducing tee has a branch smaller than the run, avoiding the extra joint of a tee plus reducer. Tees appear in supply manifolds, radiator loops, and drainage stacks.
Reducers, Couplings, and Unions
Not every joint connects pipes of the same size. Reducers step the bore up or down, couplings join two lengths in a straight line, and unions add a service point that opens without cutting. The joining method matters as much as the fitting shape, and the compression and push-fit copper fittings guide explains how mechanical joints replace soldering on metal lines.
Concentric and Eccentric Reducers
A concentric reducer is a cone with the inlet and outlet centered on the same axis. It suits vertical lines and pump suction where the flow stays centered. An eccentric reducer keeps one side flat. Installed flat side up on a horizontal line, it stops air pockets collecting at the top; flat side down, it lets condensate drain.
When to Use Each Reducer
Vertical risers and pump suction piping use concentric reducers to keep flow centered. Horizontal liquid lines use eccentric reducers with the flat side up so air can escape; horizontal steam lines use the flat side down so condensate drains. The wrong orientation traps air or water and causes noise, corrosion, or water hammer.
Couplings: Full, Half, and Reducing
A full coupling joins two pipes of the same size end to end. A half coupling is threaded or welded on one end only, branching off a vessel or a larger pipe. A reducing coupling joins two different sizes and does the same job as a reducer in a shorter length. Couplings are the default repair fitting: cut out the damaged section, slide a coupling over each cut end, and join.
Unions
A union is a three-piece fitting: two ends that thread or solder onto the pipe and a central nut that pulls them together. The joint can be opened and resealed, so unions go on both sides of pumps, valves, and water heaters. The parts are the male end, the female end, and the ring nut that clamps them.
Branch Connections: Crosses, Olets, and Swage Nipples
When a line needs more than one branch, builders choose between a cross and a series of tees, or weld an olet directly onto the main pipe. The choice changes the joint count and layout cost. On plastic systems such as PEX, branch connections follow a different logic, and the PEX piping types and fittings guide shows how manifolds replace conventional tees.
Crosses: Equal and Reducing
A cross has four openings and joins four pipes at one point. An equal cross has all openings the same size; a reducing cross has smaller branches. A cross concentrates stress at four joints and is used sparingly, mainly in fire sprinkler mains and drainage manifolds. In supply work, two tees spaced apart are easier to support and repair than one cross.
Olet Types
An olet is a branch fitting that welds, threads, or bolts onto the side of a main pipe without cutting the main. The family includes shapes named after the connection style:
- Weldolet: a butt-welded branch on a butt-welded main.
- Threadolet: a threaded branch outlet.
- Sockolet: a socket-welded branch.
- Elbolet: a branch taken off an elbow.
- Latrolet: a branch off a lateral or angled run.
- Nipolet: a threaded nipple and olet in one piece.
- Sweepolet: a curved branch that reduces stress and turbulence.
- Nipoflange: an olet with an integral flange for flanged connections.
Olets vs Standard Tees
An olet keeps the main pipe intact and adds one weld instead of two, suiting high-pressure steel systems where cutting the main would weaken it. A tee costs less labor on small plastic or copper systems and needs no welding. The decision comes down to pipe material, pressure class, and whether the main can be opened for a tee.
Swage Nipples
A swage nipple is a short fitting with two different end sizes used to join pipes of different diameters, often one end threaded and the other plain for welding. A concentric swage nipple keeps both ends on one centerline. An eccentric swage nipple keeps one side flat, mirroring the reducer choice on horizontal lines. Threaded nipples used in pairs build a union-like connection in steel piping.
End Closures, Adapters, and Valves
The last group of fittings finishes a line or changes its connection type. Caps and plugs close ends, stub ends and adapters switch joint methods, and valves control flow where fittings would otherwise sit. Material transitions need special care, and the dielectric fittings guide explains how to join galvanized and copper pipes without electrolytic corrosion.
Caps, Plugs, and Stub Ends
A cap fits over the end of a pipe; a plug fits inside it. Both close a line for pressure testing, future extension, or permanent termination, in threaded, socket, and solvent-weld versions. A stub end is a short length welded or solvent-welded to the pipe and then clamped between two flanges, which adds a flanged connection to a plain-end pipe.
Adapters
An adapter changes the joining method at the end of a pipe: from male threads to female threads, from a solvent-weld socket to threaded, or from plastic pipe to a metal fitting. Push-fit adapters convert plain PEX, copper, or CPVC ends to threaded outlets without tools. Adapters handle transitions between materials and joint systems in a building.
Valves: The Fittings That Control Flow
Valves are fittings that open, close, or regulate flow. Gate valves give straight-through flow with minimal restriction and suit on-off service. Ball valves turn a quarter turn to open fully and are the standard shutoff for branch lines. Check valves allow flow in one direction only and protect pumps and backflow-prone lines. Globe valves throttle flow and suit frequent adjustment. Install a valve wherever a line is drained, isolated, or regulated.
Choosing a fitting starts with the pipe material, then the pressure and temperature rating, then the joint method. Copper pairs with solder, compression, or push-fit joints; PEX with crimp, clamp, or expansion fittings; steel with threaded or welded joints. The basic PEX connection methods article compares push-fit, crimp, and expansion connections so installers can pick the right tool and fitting for each joint.