These days you will generally find that knitting patterns refer to knitting needles in millimetres (mm), but some patterns, and many older patterns, use traditional knitting needle sizing from the UK and USA, each of which are different. You may also find that if you get hold of some older knitting needles (I have quite a few that have been lovingly passed down from my Nannie) the number on the needle won’t be the needle’s width in millimetres, but its traditional size number. I’ve summarised the conversions for knitting needles in millimetres and traditional UK and USA sizes below.

2.00 mm1406.00 mm410
2.25 mm1316.50 mm310 1/2
2.50 mm127.00 mm2
2.75 mm1227.50 mm1
3.00 mm118.00 mm011
3.25 mm1039.00 mm0013
3.50 mm410.00 mm00015
3.75 mm9512.00 mm17
4.00 mm8615.00 mm19
4.50 mm7720.00 mm35
5.00 mm6850.00 mm50
5.50 mm59