Standard Crochet Hooks are now on sale in many sizes.  Most of these hooks are made by Susan Bates and are made of Aluminum  The only exceptions are some size C-2 which are made of QuickSilver and some size E-4 which are made of wood.
 
Standard hooks are identified with a letter, a number, and a millimeter size.  Because these sizings can vary from company to company, it's best to rely on the millimeter (mm) sizing, which is an accurate measurement.
 
Regardless of the number, letter or millimeter sizing, always complete a gauge swatch and compare it to the pattern gauge information. If your swatch is larger than the pattern gauge, redo a swatch using a smaller hook or needle. Conversely, if your gauge swatch is too small, redo it using a larger hook or needle to obtain the gauge indicated in the pattern.
 
If you don't know which hook to use, you can match your hook size to your yarn weight.
 
Size B-1 is 2.25mm.  This means that a size B crochet hook is the equivalent of a size 1 knitting needle.  Both should be 2.25mm.
Size C-2 is 2.75mm
Size D-3 is 3.25mm
Size E-4 is 3.5 mm
Size F-5 is 4mm
Size G-6 is 4.5mm
Size H-8 is 5mm
Size I-9 is 5.5mm
Size J-10 is 6mm
Size K-10.5 is 6.5mm
Size L-11 is 8mm
Size M/N-13 is 9mm
Size N/P-15 is 10mm
Size P/Q-19 is 15mm