There are three primary scales on which the size of Knitting needles are measured: the US/American, UK/English, and Metric. The US and Metric scales are by far the most common these days, and so you're only likely to encounter the UK/English scale if you've acquired a set of older needles. Nonetheless, a Needle Gauge is an invaluable tool for measuring needle sizes and performing conversions. The following table provides conversions between the different scales: