How to Measure Pipe Size

Co-authored by David Balkan

Last Updated: January 14, 2023 References

Measuring pipe size can be a little confusing at first, but anyone can learn how to do it. To find the right size, first figure out if you need to measure the outside or inside diameter, then measure it with a ruler or tape measure. Then you’ll need convert the measurement to the “nominal” pipe size, or what that pipe will be called in the store. Measuring pipe size is an important skill to have under your belt for your plumbing and building projects.

Things You Should Know

See if your pipe has "male" threads that go around the outside or "female" threads that go along the inside.
Measure the outer diameter of a pipe with male threads (or no threads) and jot down the inner diameter for a pipe with female threads.
Convert any diameter measurements smaller than 14 in (360 mm) to nominal size.
Use Nominal Pipe Size (NPS) if you live in North America and Diameter Nominal (DN) if you live in a country that uses the metric system.

Part 1

Part 1 of 2:

Measuring the Right Diameter

1
Determine if your pipe has “male” or “female” threads or no threads. Threads are the little grooves on the ends of some pipes that help them fit together. Male threads are on the outside of the pipe, whereas female threads are on the inside.^{[1]
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2
Find the outside diameter if the pipe has male threads or no threads. The outside diameter is from outside edge to outside edge across the pipe. To find it, measure around the circumference of the pipe with flexible measuring tape. Divide the circumference by pi, or about 3.14159.
- For example, if the circumference is 12.57 inches (319 mm), you would divide by pi, and get an outside diameter of about 4 inches (100 mm).
- Use string to measure if you don’t have measuring tape. Mark the point on the string where it wraps around the circumference. Then remove the string, measure it against a ruler, and divide that length by pi.
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3
Measure the inside diameter if the pipe has female threads. That is the distance across the middle of the pipe, not including the thickness of the pipe walls. Use a ruler or a caliper and measure at the end of the pipe, where there is a cross section.^{[2]
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Expert Source

David Balkan
Professional Plumber & CEO of Balkan Sewer & Water Main
Expert Interview. 29 April 2021.}
- Remember not to measure from the outside edges, but rather from inside edge to inside edge.

Part 2

Part 2 of 2:

Converting to Nominal Pipe Size

1
Convert your diameter to nominal size if its smaller than 14 inches (360 mm). If its 14 inches (360 mm) or more, you don’t need to convert it, because the diameter will equal the nominal diameter already.^{[4]
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2
Find out if you need to convert to NPS or DN. Convert to Nominal Pipe Size (NPS) if you are in North America, or Diameter Nominal (DN) if you are using the metric system.
- If you are unsure, it could help to go to the website of a store in your country that sells pipe. If they mark pipe with inches, then you need the NPS system.
3
Convert your measurement of internal or outside diameter to the appropriate nominal size. Nominal size is what the pipe will be called in the store. You can do this using a table. ^{[5]
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- This table is helpful for NPS measurements: https://www.zoro.com/pipe-fitting-size-guide
- This table has both NPS and DN measurements: https://www.massflow-online.com/faqs/where-do-nps-or-dn-stand-for/
- For example, if you measured a diameter of 1.05 inches (27 mm), this would translate to a nominal size of ¾ in NPS, or 20 in DN.

Expert Q&A

Question

How do I know what size copper pipe I have?

David Balkan
Professional Plumber & CEO of Balkan Sewer & Water Main
David Balkan is a Professional Plumber, CEO of Balkan Sewer and Water Main Service, and President of Balkan Sewer and Drain Cleaning. As a hands-on owner of these companies for over 40 years, David is knowledgeable about water service lines, sewers, and drain line issues. David is a Committee Chairman of the Master Plumbers Council and has sat on the Executive Committee of the Sub Surface Plumbers Association of New York for over 30 years. His knowledge and solution-oriented approach contributed to Balkan Sewer and Water Main Service being the largest and most trusted service in New York City and the recipient of the 2017 Angie’s List Super Service Award.

David Balkan

Professional Plumber & CEO of Balkan Sewer & Water Main

Expert Answer

Measure the outer diameter of your pipe with a caliper. Then, check online for a pipe sizing chart. Compare the caliper measurement you collected with the sizing chart, and you'll know exactly which size you're working with.
Question

What are standard pipe sizes?

David Balkan
Professional Plumber & CEO of Balkan Sewer & Water Main
David Balkan is a Professional Plumber, CEO of Balkan Sewer and Water Main Service, and President of Balkan Sewer and Drain Cleaning. As a hands-on owner of these companies for over 40 years, David is knowledgeable about water service lines, sewers, and drain line issues. David is a Committee Chairman of the Master Plumbers Council and has sat on the Executive Committee of the Sub Surface Plumbers Association of New York for over 30 years. His knowledge and solution-oriented approach contributed to Balkan Sewer and Water Main Service being the largest and most trusted service in New York City and the recipient of the 2017 Angie’s List Super Service Award.

David Balkan

Professional Plumber & CEO of Balkan Sewer & Water Main

Expert Answer

It depends on the type of pipe you're working with. Search online for a pipe sizing chart, which can point you in the right direction.