Pipe fittings come in many types and sizes, which can make shopping for plumbing projects confusing. Knowing plumbing terms, like NPT, PEX, OD, or ID, can help you determine which size or type of fitting you need. Here are plumbing and pipe fitting terms we've defined to help you become a plumbing expert.
A small door installed on a wall to provide access to a plumbing valve or connection.
A fitting to connect different types or sizes of fittings, piping or tubing that cannot be connected directly to a pipe or tubing.
Vertical air space that separates the potable water outlet of a plumbing fixture from a drain connection to prevent cross contamination. For example, a dedicated drinking water faucet with an integrated air-gap is commonly used with reverse osmosis systems. An air gap device, such as the Inline Gap-A-Flo AG150, is used to connect the drain line from a water conditioner to the plumbing waste line.
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A small 90-degree valve that transitions from household plumbing to supply lines to faucets or to toilet fixtures; can be quickly turned off in case of leaks.
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Valve that uses a lever to quarter-turn a pivoting ball inside to control fluid flow. The valve is OPEN when the lever is in-line (parallel) with the valve, thus allowing fluid to pass through the ball’s hole. The valve is CLOSED when the lever is turned 90-degrees (perpendicular) to the supply line valve.
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A type of tubing connection that pushes into the inside of a tube, typically used with softer wall tubing. Barbs hold the tubing from coming off. Some installations require adding an outer clamp to keep the tubing from coming off under pressure.
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Any part of a plumbing distribution system that is not the main, riser or stack.
British Standard Parallel Pip (straight)
A specific standardized thread type used throughout Europe, Asia (not so much in China and Japan), Australia, New Zealand, and South Africa. Straight thread version requires an O-ring or gasket to make the seal. Thread sealant not required.
British Standard Pipe Thread (tapered)
A thread type used primarily in China and Japan. Tapered with a 60-degree pitch thread (NPT has a 55 degree thread pitch) require tape or a sealing compound to ensure a good seal.
A fitting that connects plumbing or tubing through the wall of a tank or vessel.
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A plumbing fitting used to reduce or enlarge to join different pipe sizes.
A measuring device used to determine inner and outer dimensions.
A fitting used on the end of a pipe or tube to seal.
A fitting that only allows water or air flow in one direction.
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A term used for a connecting a nipple fitting with no unthreaded area.
The inner ring that acts as the grabber when tubing is inserted. When used in regard to a quick-connect or push-to-connect fitting.
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A type of threaded fitting used to connect copper, stainless steel, or plastic tubing by compressing a sleeve (ferrule) between a nut and the fitting body. Plastic tube also requires a tube insert (tube support) to prevent the pliable/flexible tube from collapsing when the compression nut and ferrule is tightened.
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A common material used in plumbing pipe and fittings that comes in various wall thickness for various water, oil, and gas applications including potable water, waste pipe, and vent pipe. Most common is type L (underground and heavy duty), M (available in both rigid and flexible forms used for inside wall home installations) , and K (rigid form used for residential water-supply lines).
A fitting that joins pipe or tubing together by gluing (PVC, CPVC), welding (steel), brazing or soldering (copper, brass etc.), or fusing (polypropylene, PVDF).
Chlorinated Polyvinyl Chloride
A household pipe material, an alternative to copper or PEX.
A plumbing fitting that joins four tubes like an intersection.
Copper Tube Size
A plumbing size classification based on the nominal pipe size or inner diameter.
The process of cleaning and smoothing the end of the pipe after cutting prior to installing a fitting.
In plumbing terms, can refer to the loss of material integrity such as corrosion.
A plumbing fitting that turns 90 degrees.
A fitting used at the end of a pipe to seal, also referred to as a cap.
Threads on the inside diameter of a fitting or pipe.
The centerpiece of a compression fitting, also referred to as a sleeve.
Female Iron Pipe
A standardized thread type; female threads are on the inside diameter of the pipe.
Any plumbing part used to join or cap pipe or tubing.
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A plumbing term that refers to faucets, shower heads, toilets, and laundry outlets. Estimating peak flow rates for a household is accomplished by adding up all fixtures.
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A pipe connection fitting that bolts to another flange; typically used with large diameter pipe 3” and above.
A type of compression fitting used with metal tubing, usually soft steel, ductile (soft) copper and aluminum, though other materials are also used. Tube flaring is considered to be a type of forging operation and is usually a cold-working procedure. Thread Sealant is not typically used with flare fittings.
An automatic valve activated by a floating ball on the end of a rod. A toilet mechanism is an example.
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Female Pipe Tapered
Standardized pipe thread type.
A sealing material used with certain fittings and plumbing fixtures. A garden hose connection uses a gasket.
Garden Hose Thread
Standard Thread type used for garden hose.
High Density Polyethylene
Tubing made from polyethylene that is stiffer and less flexible than the more commonly used LLDPE.
Type of tubing connection that pushes into the inside of a tube to hold the tubing in place, typically used with softer wall tubing. Some installations require an additional outer clamp to keep the tubing from coming off under pressure.
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Dimensional measurement used commonly with pipes. The inner diameter is also called the nominal pipe dimension to describe flow rate dimension. Copper Tube Size (CTS) is based on the inner diameter of a pipe.
A term used to describe a filter or filter housing placed in household plumbing.
Iron Pipe Thread
Standardized pipe thread type. Most commonly named by gender of the threads, MIP (male iron pipe) or FPT (female iron pipe).
Brand name for plastic material, polyvinylidene fluoride (PVDF). See PVDF.
Linear Low Density Polyethylene
Most commonly used tubing material for connecting water filtration devices. It is a bit softer and more flexible than HDPE (high density polyethylene).
A tool used with most quick-connect or push-to-connect fittings to hold tubing in place.
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Luer fittings are a standardized system of small-scale fluid fittings commonly used on medical and laboratory instruments; named after an early 20th-century instrument maker.
Threaded fitting with threads on the outside. Common types include male NPT, male compression, male BSPP, male BSPT, male iron pipe (MIP), and male flare.
Male Flared Fitting
Male Iron Pipe
A type of valve with a needle-shaped plunger that allows precise regulation of fluid flow.
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A straight pipe coupler with male threads on each end.
Nominal Pipe Size
Measurement of pipe based on the inner diameter (not actual diameter). Fitting measurement is based on nominal pipe size, so they are actually larger in diameter than their labeled size.
National Pipe Tapered
National pipe thread is tapered with a 55-degree pitch and is the most common standardized thread type used in the U.S. The tapered design mashes together when male and female parts are tightened. Thread sealant is required. When using Teflon Tape, typically only 2 turns is needed. Pipe Dope (pipe-joint compound) may also be used on metal fittings and some plastic fittings.
National Pipe Straight
Standardized thread type. Straight threads are designed to bottom out on a gasket or O-ring to make the seal.
Outer diameter of tubing. Pipe is measured by the inner diameter, which is nominally sized (i.e. CTS).
Rubber type of gasket.
The term used when a product is designed to install on board or panel, such as a pressure gauge.
A thermoplastic polymer used extensively as a tubing material for water distribution.
Refers to polyethylene tubing that has been cross-linked for additional structural strength to create pipe used mostly in household plumbing as an alternative to copper, PVC, and CPVC.
A tool used to cut pipe.
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The term used for plumbing sealant to create a watertight seal on fitting threads.
A widely used term that includes any fitting used to connect tubing or pipe.
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Each standardized thread type is designed for a specific application. Knowing the thread type of a pipe or fitting is important because fittings only mate with parts of the same thread type.
Determined by the pipe’s inner diameter (ID) to account for flow rate capacity.
A fitting used to close an open female fitting.
A plumbing device used to reduce pressure, often from the city supply before it enters the house.
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A category of fittings that connect tubing or pipe by pushing the fitting onto the tube.
Material commonly used to make plastic pipes.
Material used for fittings when chemical compatibility is needed. PVDF has a higher chemical tolerance than polypropylene and acetal plastic, more commonly used in tube fittings.
A category of fittings that allows for easy installation onto a pipe or tube (aka push-to-connect or push-in fitting).
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A category of tube fittings designed for frequent connections and disconnections with an optional built-in valve to stop the flow of fluid upon disconnecting (aka quick-couplings or interchange quick-coupling).
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A plumbing fitting that connects tubing of different sizes.
The straight part of a plumbing tee. The 90-degree outlet is called the branch.
The ferrule in a compression fitting.
The term used to describe glue and solvent pipe connections, such as PVC or CPVC.
When copper pipes are joined together by melting a soft metal alloy into the fittings.
Solenoid/ Shut-Off Valve
An electronically activated, automatic valve that controls water flow by opening or closing.
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Non-tapered threads that retain the same diameter.
An elbow fitting with male threads on one end and female on the other.
Threads that become narrow as they extend.
Common plumbing fitting that joins three tubes or pipes together. The straight part is called the run, and the 90-degree outlet is called the branch.
PTFE tape (aka plumber's tape) is a type of pipe sealant wrapped in a clockwise direction on NPT and BSPT pipe threads. Compression fittings do not require a sealant. BSPP threaded fittings use a bonded seal ring gasket to make the positive seal. Low-density white Teflon tape is used on water lines. Yellow Teflon tape is used for gas lines.
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Measurements of thread per inch of a particular thread type.
Typically based on the nominal pipe size (which is approximate inner diameter) so they are always larger in diameter than their labelled size.
Learn more about pipe dimensions and thread sizes.
Measured by foot pounds (a unit of energy equal to the amount required to raise one pound a distance of one foot), torque is the amount of force used to tighten a bolt or threaded part.
A wrench with a gauge to determine amount of torque used to tighten a bolt or threaded part.
A term used for smaller diameter plastic pipe.
Learn more about water pipes and plastic tubing.
A fitting that connects tubing in the shape of a U.
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Unified Fine Thread
A standardized thread type.
A fitting that connects two pipes of tubes.
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Unified National Special
A standardized thread type.
A plumbing fitting commonly used as an access point for drainage, featuring a 45° branch line to maintain a smooth flow.
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