How to Find the Size of a Pipe

Measure Your Supply Pipe Circumference:

Grab a piece of string about 6″(152mm) long. Strip away any insulation so you can get at the pipe and wrap the string around it. Measure how many inches of string it takes to go around the pipe once. This is the circumference of the pipe (yikes, bad memories of high school geometry!). Using the circumference we can calculate the diameter of the pipe. But school’s out so let’s forget about doing geometry calculations! Based on the string length use the table below to find your pipe size.

For Copper or PEX Pipe

2.75″ (70mm) = 3/4″ pipe

3.53″ (90mm) = 1″ pipe

4.32″ (110mm) = 1 1/4″ pipe

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5.10″ (130mm) = 1 1/2″ pipe

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For Steel Pipe or PVC Plastic Pipe

3.25″ (83mm) = 3/4″ pipe

4.00″(102mm) = 1″ pipe

5.00″(127mm) = 1 1/4″ pipe

6.00″(152mm) = 1 1/2″ pipe

For Flexible Polyethylene Pipe

2.96-3.33″ (75-85mm) = 3/4″ pipe

3.74-4.24″ (95-108mm) = 1″ pipe

4.90-5.57″ (124-141mm) = 1 1/4″ pipe

5.70-6.28″ (145-160mm) = 1 1/2″ pipe

Your string length will probably not be exactly the same as the lengths in the chart. Measurements vary a little, depending on how much the string stretches, dirt on the pipe, manufacturing tolerance of the pipe, how accurate you are at measuring, etc.