How To Measure Pipe Diameter & Size Like a Pro in 3 Steps

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If you’re planning a plumbing project or you’re going to cut into a water line to install an appliance or fixture, you’ll need to know your pipe diameter.

In this guide, we’ve shared the 3 easy steps you should take to accurately measure your pipe diameter.

📌 Key Takeaways:

  • The two measurements for pipe diameter are Diameter Nominal and Nominal Pipe Size.
  • The process of measuring pipe diameter is:
    1. Determine whether you have a female or male threaded pipe, or no threads.
    2. Find the outside diameter for male threaded pipes or pipes with no threads.
    3. Measure the inside diameter if the pipe has female threads.
    4. Convert your measurement to nominal pipe size (NPS).
  • You can use the string method if you don’t have a flexible measuring tape and you need to measure the outside diameter of a pipe.

🤔 How Is Pipe Diameter Measured?

There are two measurements for pipe diameter: Nominal Pipe Size (NPS) and Diameter Nominal (DN).

If you live in North America, you’ll need to know Nominal Pipe Size since this is how the pipes and plumbing accessories in your local hardware stores will be measured.

If you’re living in a region that uses the metric system, you need to know Diameter Nominal.

Don’t know whether to work out Nominal Pipe Size or Diameter Nominal? Pay a visit to one of your local hardware stores and ask a member of staff, or visit the store’s website and see if the pipes are sold with a measurement reading in inches. If so, the NPS system is what you need to use to measure pipe size.

Person measuring pipe diameter with a caliper

📋 Quickest Method To Find Out Pipe Size

The quickest method to find out the size of a pipe is to check the pipe itself.

There might be a spot on the side of the pipe, or on one of the valves connected to the pipe, that tells you the pipe size.

For instance, copper pipe should have a label that indicates the type of copper pipe and its NPS.

You should also find printed specifications on plastic pipes, which tells you the kind of plastic and the size. Some pipes will also share information on the pipe wall thickness.

Check for this information before you read this article any further. It could save you a lot of time.

You might not be able to read these details, or they might be in a location that’s not accessible. In this case, continue reading to learn how to measure pipe size and diameter.

📖 How To Measure Pipe Diameter: Step-By-Step Process

Read on for the step-by-step process of how to measure the diameter of a pipe.

Step 1: Determine The Pipe Thread Type

Start by determining whether your pipe has male threads or female threads.

Threads are the grooves or fittings that make a tight seal when you’re connecting multiple pipes, or a pipe to valve, fitting, or other plumbing component.

A pipe with female threads has groves on the inside of the pipe, while a pipe with male threads has grooves on the outside.

Some pipes have no threads at all.

Male and female threaded pipes

Step 2A: Measure The Inside Pipe Diameter (For Female Threads)

Move on to this step if you have a pipe with female threads.

In this case, you need to measure the inside pipe diameter. This is the distance from the inside edge of one side of the pipe to the other, not counting the pipe material (which will usually add a bit of extra length).

Measure at the end of the pipe at the cross-section, using a caliper or ruler.

Step 2B: Measure The Outside Pipe Diameter (For Male Threads Or No Threads)

If you have a pipe with male threads or no threads, follow the instructions in this step.

Measure the outside pipe diameter by measuring the distance between the outside edge of one side of the pipe to the other. To do this, use a flexible measuring tape, then divide the circumference by 3.14159 (pi) to get the measurement in inches.

So, if the pipe has a circumference of 14 inches, you could work out the outside diameter by doing the following sum:

14/3.14159 = 4.45 inches.

Handyman measuring pipe diameter

Step 3: Convert Your Measurement To Nominal Pipe Size

Finally, if the pipe diameter is smaller than 14 inches, convert the measurement to nominal size.

Most stores use nominal size when describing their products.

Don’t worry about doing this if the pipe is 14 inches or bigger because the diameter will already be equal to the nominal diameter.

You can use a nominal diameter conversion chart or table to work out your required nominal diameter based on your pipe dimensions.

The below chart shows the conversion from common diameters to nominal pipe sizes.

Nominal Pipe SizeDiameter Nominal
(in Inches)
Outside DiameterNominal Bore
1/2 1521.3015
1 1/2 4048.2640

🔁 Alternative Method: The String Method

If you have a pipe with male threads or no threads and you don’t have a flexible tape measure to measure the outside diameter, or you can’t access the open end of the pipe to measure across, you can also consider the string method to measure the pipe size.

To use this method, follow these steps:

  1. Wrap the string around the circumference of the pipe.
  2. Use a pen or marker to mark the string at the point where it meets the other end.
  3. Remove the string and place it next to a ruler. pulling it tight to get your outside diameter measurement. Measure to the nearest one-eighth of an inch.
  4. Divide the length by pi (3.14159).

Tip: If you don’t have a string, you can also use a shoelace, a ribbon, or another piece of fabric to measure pipe circumference.

📑 Final Word

Measuring pipe size is quick and easy, despite the confusing terms you might come across. You just need a few basic tools, and the knowledge of whether your pipe is male-threaded or female-threaded, or has no threads.

You might not even need to measure your pipe diameter if this information is somewhere on the pipe. Check before you get started – don’t waste your time for no reason!


How do I know what size pipe I have?

The best and most accurate way to know what pipe size you have is to measure your pipe diameter, then convert it to nominal pipe size (NPS). The exact pipe measurement method depends on whether it has male or female threads.

What’s the difference between ID and OD?

ID, or inside diameter, is the diametric distance across the pipe on the inside walls, while OD, or outside diameter, is the diameter of the pipe from one outside wall to the other.

Is pipe diameter inside or outside diameter?

You should measure the inner diameter if you have a pipe with female threads. Measure the outside diameter of the pipe if it has male threads or no threads.

  • Jennifer Byrd
    Water Treatment Specialist

    For 20+ years, Jennifer has championed clean water. From navigating operations to leading sales, she's tackled diverse industry challenges. Now, at Redbird Water, she crafts personalized solutions for homes, businesses, and factories. A past Chamber President and industry advocate, Jennifer leverages her expertise in cutting-edge filtration and custom design to transform water concerns into crystal-clear solutions.

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