Knitting Needle Size Converter

In short Knitting needles are sized three different ways and only one of them means anything absolute: the millimetre is the actual diameter, and the US and UK numbers are labels printed on the needle. Convert between them here, but the rule worth carrying away is that when a pattern gives both and they disagree, the millimetre is the one to follow. The trap is that the two numbered scales run in opposite directions. US sizes get bigger as the needle gets thicker — US 6 is 4 mm, US 8 is 5 mm. UK and Canadian sizes get SMALLER as the needle gets thicker, so UK 8 is 4 mm and UK 6 is 5 mm. Reading one scale as the other is a two-size error in the wrong direction, and the two scales collide confusingly at exactly one point: a US 8 and a UK 8 are different needles. There are genuine gaps as well. A 3 mm needle has no US size and a 3.5 mm needle has no UK one, because the scales were never designed against each other. And the US scale has one half size, 10.5, which is 6.5 mm and sits between 10 and 11. If a needle is unlabelled or the print has worn off, a needle gauge — the flat card with graded holes — settles it for very little money, and old or unbranded needles are exactly the ones whose printed size is least trustworthy.