Knitting Needles sizes are fairly straightforward in the sizes that one uses for DK, worsted weight and bulky yarns. (Size US 4 and up). But it is a much different story when it comes to small diameter needles used to knit fingering, lace and other finer weight yarns. This can be very confusing – I just discovered that there is a significant variation in the sizing labels for some of these needles.
Let’s start with the standard “US” sizing – which uses a numerical designation for knitting needles, from “0000” up to 35 – with the needle diameter increasing as the numerical designation increases. Seems like a fairly straightforward system – until you run across US size 1 & 2 needles that are different diameters. If you never use a knitting needle smaller than US size 4 (which has a diameter of 3.5 mm), this will not be a problem for you. IF, however, you get into knitting socks and lace using much smaller diameter needles – it may cause you to pull out your hair.
Fortunately, most knitting needles sold today also come with their diameter size stated as well as their US size. What I have currently in my assortment of circular knitting needles are as follows: US Size 0 (2.00 mm); US Size 1 (2.25 mm), US Size 1 (2.50 mm), US Size 2 (2.75 mm), US Size 2 (3.00 mm). These needles have all been purchased in the last 10 years. I have seen patterns referencing size 1.5 needles, so I am guessing that the size 2 (2.75 mm) needle may also be a 1.5, but it’s difficult to tell for certain. Just a word of caution to those using patterns calling for these US sizes of needles – make certain your metric diameter matches the pattern (if it’s stated) and BE SURE to check your gauge!
I found two websites that have charts comparing the US sizing to the metric diameter:
http://www.fibergypsy.com/common/needles.shtml
http://www.lionbrand.com/cgi-bin/faq-search.cgi?store=/stores/eyarn&faqKey=97
Point to remember: Gauge is the measurement of the number of stitches per inch you get with a particular needle – this is more important than using the size of needle stated in a pattern. Always do a swatch for gauge!