as OLEDOC2U said, some metric sizes do match or come close to matching SAE.
With that said, I'll add, I work as a Service Eng. for Mori Seiki Machine tool dist. I work in metric ALL the time. You can get a loose SAE on a metric bolt, but ultimitly you will "strip the bolt head",not the threads.
A trick I use; we all come across stripped boltshead, whether they be hex or allen (not including screws, using screw drivers). On our cars, rust is the reason, on my machine tools, Coolant can seize a bolt. You can step through the metric/SAE sizes to fit a stripped bolt head. With hex heads you would go down in size and with Allen head, up in size. If your not good with your eyes or math, use a micrometer/caliper to see the sizes.