How to measure pipe diameter and size.

What to know before measuring pipe sizes.

Before we walk through the steps of measuring pipes, it's important to understand nominal pipe sizes and the different types of pipes.

1. Nominal pipe size (NPS).

In order to choose a pipe that's the correct size for your project, you'll need to convert the diameter to the nominal pipe size (NPS). NPS is the North American standard pipe size, and this is the measurement you’ll most likely see at the store. NPS can be a bit confusing, but it’s a simple way for manufacturers to convey the approximate size of a pipe.

Note that NPS is different for pipes over 14 inches in diameter compared to those under 14 inches:

• Larger than 14 inches: NPS is the same as the outside diameter.
• Smaller than 14 inches: You’ll need to convert the outside diameter to NPS.

Most residential pipes will be smaller than 14 inches in diameter, so you’ll need a sizing chart to convert to NPS.

2. Types of pipes.

In addition to different sizes, plumbing pipes come in a variety of materials — and their sizes can vary. 

For example, PVC pipe sizes can differ from iron pipe sizes. However, iron and PVC pipe dimensions generally include NPS, diameter, and schedule — measurement terms we'll explore below.

3. Pipe dimensions and terms to consider.

Pipes have several dimensions you can measure. Which dimensions you need depends on the threading of the pipe.

• Pipe schedules. Pipe schedules are a numbering system that indicates the thickness of a pipe wall. Rather than an exact dimension, the schedule number indicates how thick a pipe’s wall is relative to its diameter. The schedule is typically printed on the side of a pipe, but you can also find it using a sizing guide. A schedule 40 PVC pipe with an NPS of 3 will have a wall thickness of at least 0.216 inches. A schedule 80 PVC pipe of the same NPS will have a wall thickness of at least 0.3 inches.
• Outside diameter. The outside diameter (OD) is the width of a pipe from one outside edge to the other. This includes both the thickness of the pipe wall and the width of the hole.
• Inside diameter. The inside diameter (ID) of a pipe is the width of the hole running through it. It does not include wall thickness.
• Circumference. A pipe’s circumference is the length of its outer wall if measured around its perimeter.

How to measure pipes.

Now that you understand each dimension, let’s go through the process of measuring your pipes.

What you need:

• Soft tape measure. Used to measure a pipe’s length, circumference, and diameter.
• String and marker. Used to measure circumference if you don’t have a soft tape measure.
• Calipers. Precision tool for measuring inside or outside diameter.
• Pipe sizing chart (available online). Used to convert between measurement systems.
• Gloves (optional). Protection against sharp edges on the ends of pipes.
• Pen and paper. For jotting down measurements.

Steps for measuring female fittings.

If the inside walls of a pipe have threads, it's a female fitting. Measure the pipe by following these steps:

1. Measure the inside diameter of the pipe using a tape measure or calipers.
2. Be sure to leave the wall thickness out of your measurement.
3. Convert the measurement to nominal pipe size. 

Steps for measuring male fittings or unthreaded pipes.

A male-threaded pipe has threads on the outside to fit into a female-threaded pipe. If the pipe has male threads or is unthreaded, measure it by following these steps:

1. Measure the circumference of the pipe’s outer wall with a soft tape measure.
2. If you don’t have a soft tape measure, wrap a string around the pipe. Use a marker to mark the point where the string overlaps with itself. 
3. Measure the length of the string to that mark.
4. Divide the circumference by pi (3.14159).
5. Use a pipe sizing chart to find the corresponding NPS.

Tip: If you don’t need the exact diameter of the pipe, you can use a tape measure to measure across the pipe at its widest point. This method is less accurate but will give you a decent estimate of the diameter.

Don’t want to DIY? Hire a plumber instead. 

While knowing how to measure pipe diameter and schedule is useful, some plumbing repairs aren’t suitable for DIY. For advanced repairs and installations, you may want to hire a professional.

Thumbtack is the best place to find qualified plumbing professionals in your area. Download the app today to get a free quote for plumbing services near you.