If you need to know the size of a metric male thread in this situation:
• you don't have measuring tools,
• you don't have female threads of known size to test
• you have sockets
which can happen when you lose a fastener from a vehicle and you need to buy a spare one to get home,
just slide the male thread into some sockets for hexagonal heads until you find one that matches. This works because you can inscribe a circle having a certain diameter into a hexagon as wide as that diameter and as far as I've seen the outer diameters of male threads are sligthly smaller than the nominal values and hexagonal sockets are slightly wider than the nominal value.
So for example if a male thread doesn't fit into a 9 mm socket but does in a 10 mm, then it's an M10 thread.
I guess it can work with imperial tools, too.
• you don't have measuring tools,
• you don't have female threads of known size to test
• you have sockets
which can happen when you lose a fastener from a vehicle and you need to buy a spare one to get home,
just slide the male thread into some sockets for hexagonal heads until you find one that matches. This works because you can inscribe a circle having a certain diameter into a hexagon as wide as that diameter and as far as I've seen the outer diameters of male threads are sligthly smaller than the nominal values and hexagonal sockets are slightly wider than the nominal value.
So for example if a male thread doesn't fit into a 9 mm socket but does in a 10 mm, then it's an M10 thread.
I guess it can work with imperial tools, too.