It can be challenging to figure out what size pipes you’ll need for your project. Many people believe that pipe size refers to the pipe’s outer diameter, but it actually refers to the “nominal diameter.”

Fittings can be equally perplexing. They must have inside diameters big enough to fit over the outer diameter of the pipe. This blog will assist you in determining the pipe and fitting sizes required for your next project.

At first, measuring pipe and cap size can be difficult, but anyone can learn how to do it. To determine the correct size, decide whether you need to measure the outside or inside diameter and then use a ruler or tape measure to measure it. Further, we need to convert the measurement to the pipe’s “nominal” size, which the name is given to that pipe in the store. Pipe size measurement is an important skill to have for plumbing and construction projects.

Similarly, you need to measure the pipe cap size fitting for the pipe. For instance, if you are going for 1–5/8 OD, look if it fits well with the pipe or not.

Measuring the Right Diameter

Determine whether your pipe has threads or is threadless. Threads are the tiny grooves on the edges of the pipes that allow them to fit together more efficiently. Male threads are on the outside, while female threads are on the inside of the pipe.

If the pipe has male threads or no threads, determine the outside diameter. To find it, use a flexible measuring tape to wrap around the pipe’s circumference. Subtract pi from the circumference, which equals 3.14159. If you don’t have any measuring tape, use a string. Make a note of where the string wraps over the circumference. Then take the string out, measure its length with a ruler, and divide it by pi.

If the pipe has female threads, measure the inside diameter. This is the length of the pipe in the middle, excluding the thickness of the pipe walls. Measure at the pipe’s end, where there is a cross-section, with a ruler or caliper. Remember to measure from the inside edge to the inside edge, not from the outside edges.

Converting to Nominal Pipe Size

If your diameter is less than 14 inches, convert it to a nominal size (360 mm). You don’t need to convert it if it’s 14 inches (360 mm) or larger because the diameter will already equal the nominal diameter.

Determine whether or not you need to convert to NPS or DN. If you’re in North America, convert to Nominal Pipe Size (NPS) or Diameter Nominal (DN) if you’re using the metric system.

If you’re unsure, visiting the website of a pipe and cap store in your country may be helpful. If the pipe is marked in inches, you’ll need the NPS system.

Convert your internal or external diameter measurement to the appropriate nominal size. Nominal size is what the pipe will be called in the store.

Convert Actual Diameter to Nominal Diameter

Follow these steps and use the conversion chart below to figure out what nominal pipe size you require.

For Male Threads

1. Determine your pipe or the Outside Diameter (OD) of the fitting:

  • Wrap a string around the pipe
  • Mark the point where the string meets the pipe
  • Measure the distance between the tip of the string and the mark you made (circumference)
  • Divide the circumference by 3.14159

2. Determine the nominal diameter.

For Female Threads

  1. Determine the pipe or Inside Diameter (ID) of pipe fitting (use a ruler or tape measure).
  2. Determine the nominal diameter.

Types of Fitting

There are several types of fittings for pipes. For instance:

  • Caps (for example, 1–5/8 INCH, and more)
  • Adapter
  • Flange
  • Elbow
  • Tee
  • Bushing
  • Coupling
  • Union


  • Tables can also assist you in determining your pipe’s “schedule,” related to the thickness of the wall.
  • You don’t need to transform to nominal diameter if you’re using tubing instead of piping. Tubing is based on the outside diameter.
  • The nominal diameter of PEX (cross-linked polyethylene tubing) is the same as the internal diameter.


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