UK knitting needle sizes are equivalent to the British SWG wire gauge (unless they're metric, of course).
SWG inch mm
14 .080 2.03
15 .072 1.83
16 .064 1.63
17 .056 1.42
18 .048 1.22
19 .040 1.016
20 .036 .914
21 .032 .813
22 .028 .711
23 .024 .610
24 .022 .559
25 .020 .508
26 .018 .457
27 .0164 .417
28 .0148 .376
29 .0136 .345
30 .0124 .315
