Traditional
UK sizes
Metric
US
(6)
0.6mm
14
(5)
¾mm
12
(4)
1mm
10
(3)
1¼mm
8
(2½)
1½mm
7
(2)
1¾mm
4
14 (or 1)
2mm
4
13
2¼mm
B-1, 3
(0) 
2½mm
12 (or 00)
2¾mm
 C-2, 2
11 (or 000)
3mm
10
3¼mm
D-3, 1
 
3½mm
E-4, 0
9
3¾mm
F-5, 00
8
4mm
G-6, F
7
4½mm
G, 7
6
5mm
H-8
5
5½mm
I-9
4
6mm
J-10
3
6½mm
K-10½ 
2
7mm
 
1
7½mm
0
8mm
L-11
00
9mm
M/N-13
000
10mm
N/P-15
0000
12mm
P/Q
 
16mm
Q
 
19mm
S
 
25mm
50
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Crochet Hook Transalations
(nations separated by a common language)

If you thought knitting needle conversions were tricky, then you have not seen crochet hook conversions. Here they are, as best as I can equate them.

The American knitting needle sizing could arguably be said to be more logical than the old UK sizes, in that the bigger the number the bigger the needle. Not so with crochet hooks. To quote from the "Happy Hooker", the US sizes are "a complicated numbering and lettering scheme clearly thought up by a madman" and "randomly beginning with the letter B".

The UK sizing mostly follows knitting needle sizing. Except....
When you get up to UK size 12-14, (generally the smallest within the "normal" size range), something strange happens to crochet hooks and you start counting up from zero again. These tiny steel hooks are generally used for lace making. I have a 6½ steel hook but don't know what the true conversion sizes are. You can imagine. It's small.

I have etched into my brain a memory (from 1976) of a conversation with a shop assistant in John Lewis on this very subject. When I asked if she had a "number 6" hook, she carefully explained that we were metric now and I needed 5mm; so I explained that I wanted a really tiny hook not 5mm; she said that if I wanted a metric 6mm that was really big; so I explained that the really tiny hooks were numbered differently (not at that time knowing why, nor knowing what the metric equivalent was); so she said regrettably she could not help me as they did not make any that small - at the same time making it plain that she was not only saner than I but also more knowledgeable on the matter.
And that was that.

In view of all this, and much as I spurn the metric system:

When in doubt, look for the size in mm.
[Good Advice].