Inch & Metric Thread Chart: Understanding Thread Sizes

Metric thread sizes are specified differently than inch thread sizes. Inch sizes are specified using the nominal major diameter and the number of threads per inch.

For example, a 3/8-16 (coarse) thread has a 3/8" nominal major diameter and 16 threads per inch. In contrast, a 3/8-24 (fine) thread has the same nominal major diameter and 24 threads per inch.

Understanding Inch & Metric Thread Callouts

Metric thread callouts are fundamentally different because the thread pitch is specified instead of threads per inch or per mm. The thread pitch is the distance between each thread in mm. For example, an M10x1.5 (coarse) thread has a 1.5mm pitch, while an M10x1.25 (fine) thread has a 1.25mm pitch.

To simplify metric thread callouts, the international (ISO) standard for metric threads eliminates the pitch callout on coarse threads.

For example, the thread "M10" automatically means a coarse pitch, while any added pitch callout, such as M10x1.25, specifies a non-coarse pitch. The ISO standard for measuring thread sizes eliminates the need to memorize pitch callouts. This makes it more accessible for average citizens.

Using that method, what does the designation M16x2 indicate? According to ISO standards, the pitch callout is eliminated because the thread type is coarse, so M16x2 is designated as “M16.”

ANSI Equivalents for ISO Thread Sizes

Virtually all thread standards across the world use the ISO designation convention, including the German DIN and Japanese JIS standards. However, the ANSI standard does not and asks for a pitch callout on coarse threads.

Since Carr Lane Mfg. does business across the world, we follow the international (ISO) standard for metric thread callouts. Stating the pitch of a metric coarse thread would be incorrect and totally confusing everywhere else in the world. The table below shows the ANSI equivalent callout for each ISO thread size.

What is the Difference Between Metric and Inch Threads?

Metric threads are defined as the distance between each thread in millimeters (mm). Whereas inch threads are the measure of the distance between each thread in inches.

Inch & Metric Thread Conversion Chart

The following table compares the ISO thread designation to its ANSI equivalent. If you find yourself regularly looking for equivalent thread sizes, bookmark this page or print the below chart.