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How To Measure Pipe Size: 6 Steps (With Pictures)

How To Measure Pipe Size: 6 Steps (With Pictures)

For any plumbing or building project it is important to be able to measure pipe sizes.

Unlike some materials that are quite straightforward to measure, pipes can be tricky. It’s not as simple as just picking up a measuring tape.

If you need to know how to measure pipe sizes our guide will show you how to in 6 steps. 

Different Types Of Pipes

So before we start measuring the diameter of pipes we need to understand the different types of pipes that you may come across. 

There are three main categories of pipe that we will deal with here, male pipes, female pipes and no thread. If you are a bit confused, let us explain. 

Male pipes have a thread that is on the outside of the pipe. Female pipes also have a thread but this is on the inside of the pipe. And then there are pipes that have no thread at all. 

Tapered threads are grooves that are cut into the ends of the pipes so that they can be fitted together.

If the male thread can't fit into the female fitting, then the pipes won't work together.

It is important to understand the difference between them as each of these pipes will need to be measured differently according to the pipe thread size. So let’s get to it. 

Measuring The Diameter

The two most important measurements for you to know are the outer diameter (OD) and the nominal pipe size (NPS). 

As the name suggests the outer diameter of a pipe is the width across the outside of the pipe.

The nominal pipe size (or nominal diameter) is the outer diameter cross-referenced with a pipe sizing chart. 

Step 1

Your first step is to determine whether you are measuring male, female or no thread pipes. As we said, male pipes will be identified by the threads on the outside of the rim.

Female pipes will have similar thread on the inside and no thread, well they have none. The thread dimensions will contribute to the measuring pipe size.

Of course not all pipes are the same and there are pipes made from different types of materials.

There are copper, PVC, stainless steel and cast iron pipes. But the nominal pipe size applies to all types of plumbing pipe regardless of the material that they are made from. 

How To Measure Pipe Size: 6 Steps (With Pictures)

Step 2

Now that you have identified the type of pipe you need to begin measuring.

The outer diameter of a male pipe or a no thread pipe is measured from the outside edge to the opposite outside edge across the pipe opening. 

Hold tight because this is going to get a bit mathematical. 

Using a flexible tape measure, wrap it around the pipe to get the circumference. Now you need to divide the pipe's circumference measurement by pi or 3.14159. 

As an example, if the measurement of the circumference was 12.57 inches dividing it by pi would result in a pipe outer diameter of 4 inches. 

If you don’t have a flexible tape measure, you can use the string method to measure the pipe fitting.

Wrap a piece of string around the pipe and mark where it meets the start of the string.

Then hold this against a straight ruler to get the measurement. Divide this by pi to get the outer pipe's diameter. 

Of course, you can just physically measure the end or cross-section of the pipe from outside edge to outside edge with a tape measure.

Remember that the actual outside diameter of the pipe will be bigger than its nominal pipe size. 

If you were to take just a physical measurement of a pipe and assume that was the correct diameter you would run into all sorts of problems.

The nominal pipe size will be smaller than the physical diameter that you take with a tape measure. 

This is why it is important to understand the need to convert the measurement of the outer diameter to the nominal pipe size. 

If you cannot access the end of the pipe you could use a caliper to take a direct measurement of the diameter.  

Step 3

For female pipes you measure the inside diameter. You can do this with a caliper or a ruler.

The inner diameter measurement should be taken from the inside edge to the other inside edge across the pipe’s cross-section.

This measurement does not include the wall thickness. The thickness of a pipe’s wall is referred to as the schedule.

The different schedules are 40, 80 and 120. Depending on the use of the pipe you would choose the most appropriate nominal size and schedule.

Like the nominal pipe size the schedule is not a precise measurement but rather a guide to the thickness of the pipe wall and the purpose for which it is most suited.  

Nominal Pipe Sizes

Nominal pipe sizes are what the pipes will be sold at in your local hardware store or builders merchant.

You will need to consult a nominal pipe sizing chart to convert your measurements into these sizes. 

How To Measure Pipe Size: 6 Steps (With Pictures)

Step 4

Now you need to take your measurements and convert them into nominal pipe sizes.

However this only needs to be done if the pipe is smaller than 14 inches in diameter. 

Anything above that size will be the same in terms of diameter and nominal pipe size.

It’s important to understand that the nominal pipe sizes are not actual and precise measurements, but a designation applied to pipes.

It is a way of standardizing the sizing of pipes across the country. 

Step 5

You need to find out if you have to convert your measurements to the nominal pipe size (NPS) system or the diameter nominal (DN) system.

North America uses the NPS system and the DN system is for metric measurements. 

The converted numbers will be different for each system. As the NPS is an imperial system the pipe sizes will be in inches or fractions of an inch.

It conforms to the American Society of Mechanical Engineers (ASME) standards.

The DN system is a metric system and conforms to the International Organization for Standardization (ISO) metric standards.

This applies to all piping in plumbing, heating oil, natural gas, and other types of piping.

Step 6

Now you can convert your internal or outer diameter measurements to the appropriate nominal size.

This is the size that the pipe will be known as in your local plumbers merchant or building supplies store. 

There are many tables available online or on the websites of pipe manufacturers that you can use for converting your measurements. 

So in order to determine the nominal pipe size or diameter nominal size you need to consult a conversion chart.

When you look at these charts you can see the difference between the actual outer diameter measurement and the nominal pipe size. 

Final Thoughts

There are many different types of pipes that you may need to measure such as PVC pipes, copper, or heavy-duty sewage pipes.

It is an important skill to be able to accurately measure the size of pipes that you need for your project. 

The process may seem complicated but once you understand the way pipes are sized and how that relates to their physical dimensions it becomes a lot easier to understand. 

We hope this guide to measuring pipes has been helpful and that you will now be able to measure with confidence for all your projects. 


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