The M4 machine screw has a thread diameter of 4 millimetres, in simple terms the M stands for millimetres followed by the size – in this case 4 mm.
What is the diameter of an M4 thread?
Tap size Basic major dia (mm) Basic major dia (inch)
----------- -------------------- ----------------------
M2.5 x 0.45 2,5mm .0984
M3 x 0.5 3mm .1181
M3.5 x 0.6 3,5mm .1378
M4 x 0.7 4mm .1575
At the threaded bit, an M4 would be 4mm across. ... It's a bit more accurate than that, everything from the diameter of the bolt without the thread to the shape of the thread is defined to a tolerance.
In metric nomenclature, an M4 x 0.7 cap screw has a 4 mm barrel and the pitch is 1 thread per 0.7 mm. (The term M4 x 0.7 is often shortened to just M4.)Apr 23, 2021