Crochet Hook Sizes: mm, US Letters and UK Sizes

A pattern says “size H hook”, your hook says 5 mm, and an old one in the drawer says 6. They're all the same hook. Our hook size converter shows every system side by side.

How to read hook sizes

• Millimetres are the most reliable: they measure the hook's shaft and are printed on almost every modern hook.
• US letters come with a number (H-8 is 5 mm). Brands can differ by a fraction of a millimetre.
• Old UK and Canadian sizes run backwards: a bigger number is a smaller hook.
• Steel hooks are for thread, and their numbers also run backwards.

Which hook for my yarn?

The converter shows the yarn weight each hook suits. As a guide: DK (3) uses about 3.5–4.5 mm, worsted (4) about 4.5–5.5 mm and chunky (5) 5.5–6.5 mm. For amigurumi, go a size or two smaller so the stuffing doesn't show.

If your piece comes out the wrong size, the hook is usually the fix: the gauge calculator tells you which size to try.