Sizes given to pipes and pipe fittings can be confusing to DIYers because there are many different ways to size a pipe. IPS, or iron pipe size, is one of them. The IPS standard dates back to the early 19th century and was originally used for sizing wrought iron pipe. Today, IPS and its related thread-size standard, NPT (National Pipe Thread), are found on most threaded plumbing parts in hardware and home stores, including iron, steel, brass, and PVC parts. Copper and PEX pipe use a different system, called copper tubing size (CTS). The two different sizes are not compatible, but you can make connections between IPS and CTS parts using special adapters.
Tip
An IPS connection refers to the pipe and thread sizing standard (iron pipe size) used on various pipe materials and fittings.
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IPS in Other Forms
When looking for threaded plumbing fittings, you often see the terms MIPS and FIPS. These are based on IPS and simply note that the fitting is a male fitting (MIPS) or a female fitting (FIPS). And some parts have both. For example, a 1-inch, threaded 90-degree elbow may read 1-inch FIPS x 1-inch MIPS 90 degree. This indicates a 1-inch (IPS) elbow with a 1-inch female-threaded end and a 1-inch male-threaded end, and the elbow makes a 90-degree turn. The size and type of each end are specified because these can vary. For example, another elbow might have a 1-inch MIPS end and a 1 1/2-inch FIPS end.
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IPS and NPT
The standard for threading IPS pipe is known as National Pipe Tapered Thread (NPT) and is often used interchangeably with IPS. This standard assures that no matter what type of fitting you need, if both parts are labeled IPS or NPT, they will fit together. For example, if you need a 1-inch IPS threaded pipe to fit in a 1-inch threaded NPT elbow, the connection will fit together.
- MPT: Male Pipe Thread
- FPT: Female Pipe Thread
- FIP: Female Iron Pipe
- MIP: Male Iron Pipe
Using the 1-inch pipe as an example again, a 1-inch FPT x 1-inch MPT 90 degree (1-inch female pipe thread by a 1-inch male pipe thread at 90 degrees) will fit and make a watertight seal after applying the proper sealant and tightening to the specified torque.
Importance of IPS and NPT
One of the most common applications of NPT is in a hydronic heating system. All modern-day boilers, water heaters, pressure regulators, and gas and oil fittings, along with other various accessories, have NPT connections. Remember that these NPT connections are based on IPS dimensions. You'll also find IPS and NPT and related sizing standards on galvanized steel water pipe and fittings, brass shutoff valves, washing machine hoses, water supply hoses (such as faucet connectors), and many other household plumbing items.
IPS and PVC
PVC (polyvinyl chloride) pipe and fittings also use the IPS standard, even though they're plastic and are often glued together. While many people think IPS relates only to threaded fittings or pipe (since iron pipe is typically threaded), the IPS sizing standard is more about the outer dimension of the pipe. The outside diameter of most PVC pipe is the same as the outside diameter of steel pipe of the same nominal size. PVC fittings can also be threaded, and these follow the NPT sizing standard.