Inch & Metric Thread Sizes Explained

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, while 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 (distance between each thread, in mm) is specified instead of threads per inch or per 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. This method of designation has the major benefit that average citizens do not need to memorize pitches just to identify a coarse thread.

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. All except the ANSI standard, which so far still asks for a pitch callout on coarse threads, despite the confusion that it causes.

Since Carr Lane 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 following table shows the ANSI equivalent callout for each ISO thread size.

 

ISO THREAD DESIGNATION ANSI EQUIVALENT
M1.6 M1.6x0.35
M2 M2x0.4
M2.5 M2.5x0.45
M3 M3x0.5
M3.5 M3.5x0.6
M4 M4x0.7
M5 M5x0.8
M6 M6x1

M8

M8x1

M8x1.25

M8x1

M10

M10x1.25

M10x1.5

M10x1.25

M12

M12x1.25

M12x1.75

M12x1.25

M14

M14x1.5

M14x2

M14x1.5

M16

M16x1.5

M16x2

M16x1.5

M18

M18x1.5

M18x2.5

M18x1.5

M20

M20x1.5

M20x2.5

M20x1.5

M22

M22x1.5

M22x2.5

M22x1.5

M24

M24x2

M24x3

M24x2

M27

M27x2

M27x3

M27x2

M30

M30x2

M30x3.5

M30x2

M33

M33x2

M33x3.5

M33x2

M36

M36x3

M36x4

M36x3

M42 M42x4.5
M48 M48x5
M56 M56x5.5
M64 M64x6
M72 M72x6
M80 M80x6
M90 M90x6
M100 M100x6