7 crochet hook: 7 is 4.5 mm

A US size 7 crochet hook is 4.5 mm at most brands. Turned around: a hook stamped 4.5 mm is usually sold as a 7. In the old UK numbering it is a 7.

The letter and the number are one hook, not two — “7” is a single size. Older hooks often print only one of the pair, which is most of the confusion. The millimeter is stamped on the hook itself, so when a pattern and a label disagree, trust the mm.

What yarn this hook is for

4.5 mm falls in the hook range the Craft Yarn Council suggests for 2 yarn weights — guidelines, in their own words, not rules:

• Fine (2) — also sold as Sport, Baby. Expect about 16–20 single crochet to 4 inches.
• Light (3) — also sold as DK, Light Worsted. Expect about 12–17 single crochet to 4 inches.

Before you trust the letter

The Craft Yarn Council puts it plainly: “Because letter and number sizing vary from company to company, rely on the package millimeter (mm) sizing, which is an accurate measurement.” Only E (3.5 mm) and H (5 mm) mean the same thing at every brand — a hook stamped G is 4 mm from Susan Bates or Clover and 4.25 mm from Boye, and from L upward the letters drift far enough that the Council stops printing single ones.

The UK number is history rather than shopping: British hooks have been sold in millimeters since the 1970s, and these numbers turn up in vintage patterns and on inherited hooks. Watch one trap — UK crochet hooks and UK knitting needles are numbered differently below 4 mm, and a great many charts online quietly print the needle ladder for hooks. Vintage steel thread hooks used a third scale again.

The sizes either side

• One size down: 4.25 mm
• One size up: 4.75 mm

Half a millimeter is enough to change how a finished piece comes out. If your gauge is off, this is the dial to turn before you change anything in the pattern — the gauge calculator says which way to go.

See every hook size in one chart →