Why two numbering systems?
Knitting needles are measured in two ways. The simplest is the diameter in millimetres, used almost everywhere in Europe: a 4 mm needle is 4 mm across, and that is all. American patterns use a number instead, from 0 to 50, which means nothing by itself: "US 8" says little until you match it to its millimetre value, here 5 mm. The bigger the number, the bigger the needle.
A third numbering, British, was used for a long time. It counts backwards, with a smaller number for a bigger needle, and it has almost vanished from recent labels. We do not reproduce it in the chart for lack of a verified source: on an old pattern, look for the diameter in millimetres when one is given.
Reading "US 8" in a pattern
When a US pattern says "size 8 needles" or "US 8", it means the American number, not millimetres. Type it into the finder above with the "US" prefix: without it, a bare number is read as millimetres first, because "8" is also a diameter in the chart (8 mm, which is US 11). Only size 10½ is written with a half, and it matches 6.5 mm.
The chart on this page follows the correspondence published by the Craft Yarn Council, the American body that standardises yarn labels and patterns. It covers nineteen sizes, from 2 mm (US 0) to 25 mm (US 50). It describes what American patterns expect, not a law: two brands can differ by a fraction of a millimetre, and the diameter stamped on the needle remains the only dependable figure. Measure it with a gauge if the marking has worn off.
Straight, circular, double-pointed
Needles come in three shapes, and the size does not change from one to the other: a 4 mm circular needle makes the same stitches as a pair of 4 mm straight needles. What changes is the use. Straight needles suit small pieces worked flat, such as a scarf. Circular needles, two tips joined by a flexible cable, carry the weight of a large piece on the cable rather than on your wrists, and can also be worked in the round for a seamless sweater. Double-pointed needles, sold in sets of four or five, are for small tubes: socks, sleeves, the crown of a hat.
A pattern worked in the round therefore often calls for two kinds of needles of the same size: a circular one for the body, a double-pointed set for the end of the decreases. Check the size of each before you start.
Choosing your size from the yarn and your gauge
A yarn label suggests a needle, often as a range in millimetres, and it is a good starting point. But the right size is the one that gives the right tension with your hand and your yarn, and that cannot be guessed: it has to be measured on a swatch. Knit a square of about fifteen centimetres in the pattern’s stitch, wash it and let it dry the way the finished piece will be treated, then count the stitches and rows over ten centimetres.
Too many stitches over ten centimetres: your work is too tight, move up to a bigger needle. Too few: it is too loose, take a finer needle. Half a size already makes a clear difference to a garment. The gauge calculator does the comparison for you, and tells you how far you are from the pattern. For a piece that must stay firm, a bag for instance, it is fine to knit a little tighter than the label suggests.
And if you come across a crochet pattern?
Crochet hooks follow another scale, with letters and numbers: the crochet hook size chart gives their millimetre equivalents. The same diameter does not suit both crafts, however, since the yarn is not handled the same way.