Crochet Hook Sizing Chart

Here's a helpful crochet hook size chart that breaks down the millimeter (mm) size vs. US number and letter size (#). 

MM Hook Size US # Hook Size
 2mm #0
2.25mm
#B-1
2.75mm
#C-2
3.25mm #D-3
3.5mm #E-4
3.75mm #F-5
4mm #G-6
4.5mm #7
5mm #H-8
5.5mm #I-9
6mm #J-10
6.5mm #K-10.5
7mm #10.75
8mm #L-11
9mm #M/N-13
10mm #N/P-15
12mm #16
12.75mm #17
15mm #P/Q-19
19mm #S-35
25mm #50