Why a letter and a number?
American crochet hooks carry two markings: a letter and a number, as in "G-6". The letter follows the diameters in increasing order, from B to P, and the number gives the size’s position in a second series. Both say the same thing, and a US pattern often quotes only one of them: "size G" or just "6". The only size in the chart with no letter is the 7, which is 4.5 mm.
Manufacturers have not all applied these markings equally strictly, and two hooks with the same letter can differ by a quarter of a millimetre depending on the brand. That is why the diameter in millimetres, stamped or printed on the handle, remains the only dependable figure. Measure your hook with a gauge if the marking has worn off, and trust that rather than the letter.
The Craft Yarn Council standard
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 fourteen sizes, from 2.25 mm (B-1) to 10 mm (N/P-15). The two-letter notations, "M/N-13" and "N/P-15", exist because manufacturers disagree on the letter for these large sizes: the number does not change.
This standard is a reference, not a law: it describes what American patterns expect, and that is why it is used to translate a pattern. It says nothing about steel hooks, the very fine ones reserved for lace, which have their own numbering and are not covered here.
Millimetres, US and the British numbering
The millimetre is the common language: most hooks sold in Europe show only that, and a French or German pattern always speaks in millimetres. The American system is the one with letters. There is also an older British numbering, which counts backwards: the bigger the number, the finer the hook. It has almost vanished from recent labels, and we do not reproduce it here for lack of a verified source: if you are unsure about an old pattern, look for the diameter in millimetres.
Choosing your size from the yarn and your gauge
A yarn label suggests a hook, 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. Work a square of about ten centimetres in the pattern’s stitch, count the stitches and rows over ten centimetres, and compare with what the pattern states.
Too many stitches over ten centimetres: your work is too tight, move up to a bigger hook. Too few: it is too loose, take a finer hook. Half a millimetre already makes a clear difference to the size of a garment. A tight hand more often needs a bigger hook than the pattern’s, and the opposite for a loose one. A piece that must hold its shape, such as an amigurumi, is often worked with a finer hook than the label suggests, for a dense fabric that does not let the stuffing show.
What to do when the pattern says "size G"
A letter on its own means the matching US size: "size G" means G-6, which is 4 mm. Type the letter into the finder above to confirm it. Beware of shared letters, M and N for example: if the pattern quotes a number, prefer it to the letter. And if your pattern quotes a size that is not in the chart, do not round it at random: check the original pattern or ask the designer.
For a whole pattern there is no need to convert every size by hand: the US ↔ UK converter adds the equivalent next to each hook size it finds, at the same time as it translates the abbreviations.