Crochet hook size converter

Hook sizes are labelled in several systems, and they do not all agree. A vintage UK pattern's "size 8", a US "G-6" and a Japanese "7/0" are all 4 mm, but a steel "size 7" for thread is about 1.5 to 1.65 mm. This converter looks up a size in any system and shows the others, with the brand differences you need to know.

How to use the converter

1. Choose the system the size is written in: millimetres, US, old UK or Canadian, US steel or Japanese.
2. Type the size, for example 4, G-6, 8, 7 or 7/0.
3. Read the table. Warnings appear when a size means different things in different systems.

How the converter works

Millimetres are the only measured size, so every lookup goes through the diameter. There is no formula: the converter matches your size against a table that follows the Craft Yarn Council hook chart, with known brand variations. A millimetre size that is not on the chart is matched to the nearest standard hook.

• Boye labels a 4.25 mm hook G-6, while Susan Bates and Clover G-6 hooks are 4 mm. Boye also makes 5.25 mm (I) and 5.75 mm (J) hooks.
• 9 mm is sold as M-13 or N-13, and 10 mm as N-15 or P-15.
• Some sizes, such as 2 mm, 3 mm and 7 mm, have no official US letter.

Steel hooks for thread run the other way, so a bigger number is thinner, and they vary by brand, so each is shown as a range.

Reading your results

• Millimetres, US and old UK: the sizes in each system.
• Regular hooks table: the matching row with the Japanese size and brand notes, plus the yarn weights the size usually suits.
• Steel hooks: listed for small sizes, and when a US number could mean a steel hook.
• Warnings: when a size such as "7" could be a regular 4.5 mm hook or a steel hook.

Worked examples

• 4 mm: US G-6, old UK 8, Japanese 7/0. Usually used with light and medium (DK and worsted) yarn. Note the Boye 4.25 mm G-6.
• 3 mm: no official US letter, old UK 11 or 10, Japanese 5/0.
• US 7: a regular 4.5 mm hook, or a steel size 7 (about 1.5 to 1.65 mm) for fine crochet thread.

US and UK sizes

Old UK and Canadian numbers came from wire gauges, so they get smaller as the hook gets bigger: UK 14 is 2 mm, UK 4 is 6 mm and UK 000 is 10 mm. Modern UK hooks are labelled in millimetres.

Accuracy and limits

• The table is checked by our automated tests. Brands differ by fractions of a millimetre, so the millimetre size printed on your hook or pattern always wins.
• Steel hook sizes vary most between brands; each steel size is a range.
• Old hooks can be off from their label. Check an unlabelled or vintage hook with a needle and hook gauge.

Common mistakes

1. Buying by letter alone. A G-6 from one brand can be 0.25 mm bigger than from another. Buy by millimetres.
2. Confusing a US 7 with a steel 7. One is 4.5 mm, the other about 1.5 mm.
3. Reading old UK numbers as millimetres. A UK 8 is 4 mm, not 8 mm.

Tips for more accurate results

• When a pattern gives both a letter and millimetres, follow the millimetres.
• Unsure which hook a yarn needs? The yarn weights guide linked below has the usual pairings; then swatch.
• Changing hook to match gauge? The swatch troubleshooter suggests the next size.

Frequently asked questions

Related guides and tools

• Which hook suits which yarn weight