Question about crocheting doilies?

I like to crochete with regular yarn and my friend gave me a box today with instruction books and yarn to crochete doilies, which I have never done before but would like to try, there was also a hook in the box, the hook only has a letter for a size (letter F) and the instructions all give numbers for the size hook you need, is there a chart that shows the number equivalant of the letter on the hook? Like on my other regular hooks it says U.S 6G/4.00 MM