As per my Metalworkers Data Book Ian, the values are-
M3. 2.8mm
M4. 3.7mm
M5. 4.6mm
M6. 5.6mm
I have been known to up M5 and M6 to 4.7 and 5.7 where load on the screw was not that great, or where depth of engagement was at least twice the diameter. They do though benefit from lubrication of some form, but not cutting oil.
The data book also includes other metric sizes as well as Unified fine and course, also BA sizes.
If I can give the book a plug, 220 pages of data for just over £5 is the best value book in the Workshop Practice Series, size wise that is. See here for more details.
Harold