Knitting Needle & Crochet Hook Sizes

A size 13 needle is either 9mm or 2.25mm

US knitting numbers go up as the needle gets thicker. Old UK and Canadian numbers go down. Both are strictly monotonic, in opposite directions, so nearly every number that exists in both systems means two different needles — and at the ends of the range they are four times apart.

A vintage or British pattern asking for a size 13 wants a 2.25mm sock needle. A US knitter reaching for their size 13 picks up a 9mm. That is not a gauge problem you can swatch your way out of; it is a different craft.

…except for exactly one number

Sweeping every number present in both systems turns up precisely one that refers to the same needle in each: 7, at 4.5mm. The two scales run in opposite directions and happen to cross there. It is a coincidence rather than a design, and it is the only needle number you can quote without saying which system you mean. Every other shared number is a trap.

"Go up one size" is not a constant instruction

The metric steps between consecutive sizes run from 0.25mm to 6mm — a 24-fold range — while the numbering treats every one of them as a single step.

Stitch width scales with the needle's diameter, so gauge goes as 1/d. That means one step changes gauge by anywhere from -6.3% to -24.0% — a factor of 3.8 between the gentlest "one size" and the harshest. Advice of that form means very different things depending on where you happen to be standing.

That model is worth trusting for one reason: it was checked against something independent. It predicts 0.31 to 0.56 stitches per inch per step across the ordinary garment range, and all ten of those steps land inside the quarter-to-half-a-stitch band this site had already recorded from practice on its gauge calculator. A derived model reproducing an empirical one is the only reason to prefer it to a lookup.

Full conversion table

The blanks are real and load-bearing. 3 metric sizes have no US number at all (3mm, 7mm, 7.5mm) and 5 have no UK number (3.5mm, 12mm, 15mm, 19mm, 25mm). A converter that snapped those to the nearest number instead of saying so would be quietly lying.

Crochet hooks are worse, and it is worth knowing where

US hooks carry a letter and a number, two parallel systems that do not always agree, and above about 9mm the lettering stops being dependable at all — the same hook is sold as M by one manufacturer and N by another, and the next size up as N or P. Those rows are marked "varies" above rather than picking a side, because there is no side to pick.

Millimetres are the only figure that means anything. If a hook has both stamped on it, believe the millimetres. If a pattern gives only a letter, take its stated gauge as the real instruction and swatch for it.