Free crochet tool
Crochet Hook Size Converter (mm, US, UK)
A crochet hook size converter translates between metric millimetres, US letter-number sizes and old UK numbers. Enter any one of them to find the equivalents. Millimetres are the reliable measure, because US letters were not always consistent between brands and some metric sizes have no US or UK equivalent.
By the crochetpatterns.io editorial team · Reviewed · Editorial policy
Result
| mm | US | Old UK |
|---|---|---|
| 2.25 | B-1 | 13 |
| 2.75 | C-2 | 12 |
| 3.25 | D-3 | 10 |
| 3.5 | E-4 | 9 |
| 3.75 | F-5 | — |
| 4 | G-6 | 8 |
| 4.5 | 7 | 7 |
| 5 | H-8 | 6 |
| 5.5 | I-9 | 5 |
| 6 | J-10 | 4 |
| 6.5 | K-10½ | 3 |
| 7 | — | 2 |
| 8 | L-11 | 0 |
| 9 | M/N-13 | 00 |
| 10 | N/P-15 | 000 |
| 11.5 | P-16 | — |
| 12 | — | — |
| 15 | P/Q | — |
| 16 | Q | — |
| 19 | S | — |
| 25 | T/U/X | — |
Why do crochet hooks have different size systems?
US hooks use a letter and number, such as H-8 for 5 mm. Old UK and Canadian hooks used numbers that get smaller as the hook gets bigger. Modern patterns everywhere increasingly list millimetres first.
Always trust the millimetre size printed on the hook shaft. A few brands label 6.5 mm as K-10½ and others as K-10, so letters alone can mislead.
Which hook size do I need for my yarn?
The yarn label lists a recommended hook. As a guide, DK / light worsted (weight 3) suits 4.5–5.5 mm and worsted (weight 4) suits 5.5–6.5 mm. Amigurumi is usually worked 0.5–1 mm smaller than the label for a tight fabric.
Frequently asked questions
What is an H hook in mm?
An H-8 hook is 5 mm.
What is a 4 mm hook in US sizes?
4 mm is a G-6 hook in US sizing.
Are steel hooks sized the same way?
No. Steel thread hooks use a separate numbering where larger numbers are smaller hooks; check the millimetre size.