Simple metric short-cut
You can quickly get a 'ballpark' conversion from English to Metric if you remember this rule of thumb:
One 'mm' is about one-and-a-half 'sixteenths'.
If that seems like a mouthful, here is an example:
The nut you need to loosen looks like a 9/16ths wrench might fit. But you try it and the wrench doesn't fit the nut just right. Maybe the nut is Metric. Ok, which Metric wrench should you try next? Well, half of 9 is 4½. Add that to the 9 and you get 13½.
One-and-a-half of 9 is 13½ (9 x 1.5 =13.5).
So, grab a 13mm or a 14mm wrench.