Metric Pipe Thread Size Guide


Most of the world uses the metric system to measure pipe sizes. However, America, Liberia and Burma still use the Standard English system. Converting pipe size from standard to metric can be confusing but knowing them can be useful in day-to-day pipe work.

Under an Inch

  • An eighth of an inch is equivalent to six millimeters, while three sixteenths is equal to seven millimeters. One quarter of an inch is eight millimeters and three eights is 10. Half an inch is 15 millimeters, five eighths is 18 and three quarters is 20 millimeters.

One Inch

  • One inch is the same as 25 millimeters. Another quarter inch equals 32 millimeters. An inch and a half is the same as 40 millimeters. Pipe measurements don’t come in any more sizes that are less than two inches.

Two Inch

  • Adding another inch to your first inch will give you 50 millimeters. Two and a half millimeters is equivalent to 65 millimeters. There are no more metric pipe measurements that are below three inches.

Three Inch

  • The synchronization between millimeters and inches is not perfect. For example, three inches is not 75 millimeters, as you might expect, but 80 millimeters; three and a half inches are 90 millimeters.

Four Inch

  • Four inches equals 100 millimeters. This is another moment of non-perfect interaction between the systems. The next measurement less than five inches is four and a half inches, which equals 115 millimeters.

Five and Over

  • The last two pipe measurements are more uniform to early conversion rates. Five inches is 125 millimeters while six inches is 150.

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