Metric fasteners differ drastically from their US counterparts and are absolutely not interchangeable. This is most apparent in how Metric screws use a Thread Pitch where US screws use 'Threads Per Inch' (TPI). Metric Thread Pitch is calculated as the millimeter (mm.) distance between two thread peaks. For Thread pitch, the smaller the number the finer the threads. Metric Thread Pitch diagramUsing the M8 screw example above, the screw has a Thread Pitch of 1.25mm. You might find that counting the distance between 10 threads and then moving the decimal point over one space is much easier than trying to accurately measure the spacing of one thread.

Screw Size Metric Thread Pitch (mm.)
Standard / Coarse Fine
M1.6 0.35 -
M2 0.40 -
M2.5 0.45  -
M3 0.50  -
M3.5 0.60  -
M4 0.70  -
M5 0.80  -
M6 1.00  -
M8 1.25 1.0
M10 1.50 1.25
M12 1.75 1.50
M14 2.00 1.50
M16 2.00  
M18 2.50  
M20 2.50