Below is a needle size conversion chart comparing the most common systems - metric, US and UK. Metric is becoming the most well-known since it makes the most sense - sizes are determined by the diameter of the needle in millimeters (mm). North American needles usually have both US and metric sizes indicated on the needle or the packaging.
It is also useful to have a needle gauge accessory like the Susan Bates Knit-Chek below. Printing often gets worn off wooden and bamboo needles and DPNs (double-pointed needles) usually don't have the size on the needle. Just see what size hole your needle fits through - it gives metric and US size. This handy tool also has centimeters and inches for checking your knitting gauge.
