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How to Measure Pipe and Fitting Sizes

Determining what size pipes you need for your project can be confusing. Many people assume pipe size is the outer diameter of the pipe—but “pipe size” actually refers to what is called “nominal diameter.”

Fittings can be just as confusing. Their inside diameters must be large enough to fit over the pipe's outside diameter. A half-inch plastic elbow, for example, has an outside diameter of about 1-1/4 inches.

Use this guide to help you find the pipe and fitting sizes you need for your next project. (For help with sizing PVC, download our PVC and CPVC Pipes and Fittings Size Guide.)

EZ-TIP 1: Convert Actual Diameter to Nominal Diameter

The easiest way to find what nominal pipe size you need is to use follow these steps and use the conversion chart below.


For Male Threads

  1. Measure the Outside Diameter (OD) of your pipe or pipe fitting:
    • Wrap a string around the pipe
    • Mark the point where the string touches together
    • Use a ruler or measuring tape to find the length between the tip of the string and the mark you made (circumference)
    • Divide the circumference by 3.14159
  2. Use the chart on this page to find the nominal diameter (pipe size).

For Female Threads

  1. Measure the Inside Diameter (ID) of your pipe or pipe fitting (use a ruler or tape measure).
  2. Use the chart on this page to find the nominal diameter (pipe size).

Nominal Diameter Conversion Chart

(All Measurements in Inches)

Outside or Inside Diameter Decimal Equivalent Nominal Diameter Typical Threads Per Inch
5/16 0.313 1/16 27
13/32 0.405 1/8 27
35/64 0.540 1/4 18
43/64 0.675 3/8 18
27/32 0.840 1/2 14
1-3/64 1.050 3/4 14
1-5/16 1.315 1 11-1/2
1-21/32 1.660 1-1/4 11-1/2
1-29/32 1.900 1-1/2 11-1/2
2-3/8 2.375 2 11-1/2
2-7/8 2.875 2-1/2 8
3-1/2 3.500 3 8
4 4.000 3-1/2 8
4-1/2 4.500 4 8

EZ-TIP 2: Pipes vs. Tubing

Pipe and tubing are not measured the same way. Tubing is measured and named based on the actual outside diameter of the tube.

PEX, or Cross-Linked Polyethylene Tubing, is another technology fast becoming popular, and it is measured and named by inside diameter.


Pipe vs. Tubing Outside Diameter
1/2” size pipe 27/32”
1/2” size tubing 1/2”

EZ-TIP 3: Select Thread Type

One of the most common thread types is National Pipe Thread (NPT). These come in male (NPT, MPT, or MNPT) and female (FPT or FNPT). These are usually tapered threads used in joining and sealing pipes.

Another common thread is National Standard Free-Fitting Straight Mechanical Pipe Thread (NPSM). These straight thread pipes are typically used for mechanical joints.

BSP thread form stands for British Standard Pipe. It is based on trade size rather than actual diameter.

To learn more about pipe thread types, download our Pipe Thread Definitions eZTip.

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Still Need Help?

If you are still having trouble choosing pipe or fittings, contact our Customer Service Center at or call 855-289-9676.

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