UK needle sizes (which are still used by those, like myself, who learnt to knit before metric sizing was introduced in the 1980s – we can convert readily from one system to the other) are based on the Imperial or British Standard Wire Gauge (SWG). But, curiously enough, US/Canadian needle sizes are not based on the American Wire Gauge. In fact, as far as I can tell from a quick bit of research, nobody is really sure where the numbering system used in North America came from.
What led me to consider needle sizes was a pair of large wooden needles that someone in my knitting group wanted rid of.
Last winter I knitted a couple of hats in thick yarn, for which I had to swap between my largest interchangeable points (8mm) for the ribbing and 10mm single-pointed needles (SPNs) for the rest of each hat. For some reason, I have several pairs of 10mm SPNs but I couldn’t find any 8mm or 9mm ones. So, when no one else wanted this pair of large, number-less needles, I took them in the hope that they would turn out to be a size I was missing – none of us had a gauge with us that went up to the really large sizes.
When I got home I tried the needles in my gauge, which has metric sizes printed on one side and UK ones on the other. They went into the 10mm (UK 000) slot with ease, indeed they rattled around in there, but they wouldn’t fit into the 9mm (UK 00) slot.
Now, I have always known that UK needle sizes conform to SWG because my father, who worked in the steel industry, could accurately tell the size of a DPN with no markings on it just by feeling it – a useful skill when neither my mother nor I happened to have a gauge to hand. So I went to look up the SWG sizes and found that 000 is just under 9.5mm whereas 0000 is just over 10mm.
Clearly, these unmarked wooden needles are size 000 and I have belatedly realised that my needle gauge is actually a metric one with nominal UK equivalent sizes printed on one side.
Not that it makes much difference in the smaller sizes, but it might matter when trying to decide which large needles to try first to achieve the correct tension, because the gaps between easily-available metric needle diameters are a whole mm once you get above 7 or 8mm.
I did consider broaching my new 000s down to 9mm – I have a steel drill gauge with a 9mm hole that I made during my engineering training which would probably do the job if I bashed each needle though it with a mallet – but I have decided against it. I often knit from old patterns that call for the UK needle sizes and, in any case, when you are knitting thick yarn on large needles you don’t need many stitches and therefore a small tension discrepancy makes little difference.