Crochet hooks come in all shapes and sizes. The United States (US) and the United Kingdom (UK) use different sizing. Here is a chart of US and UK sizes with their corresponding sizes based on the metric system, so you know which hook to use when following a pattern that uses US, UK or metric sizes.
| Millimeter sizes (metric) | Size in the US in plastic/wood etc or steel | Size in the UK in plastic/wood etc or steel |
| 1.5 mm | 6 steel | 3.5 steel |
| 1.75 mm | 5 steel | – |
| 2 mm | 4 steel | 14 or 2.5 steel |
| 2.25 mm | B-1 | 13 or 1.5 steel |
| 2.50 mm | – | 12 |
| 2.75 mm | C-2 | 1 steel |
| 3 mm | – | 11 |
| 3.125 mm | D | – |
| 3.25 mm | D-3 | 10 or 0 steel |
| 3.50 mm | E-4 | 9 |
| 3.75 mm | F-5 | 8 |
| 4 mm | G-6 | – |
| 4.25 mm | G | 7 |
| 4.50 mm | 7 | – |
| 5 mm | H-8 | 6 |
| 5.25 mm | I | – |
| 5.50 mm | I-9 | 5 |
| 5.75 mm | J | – |
| 6 mm | J-10 | 4 |
| 6.50 mm | K-10.5 | 3 |
| 7 mm | – | 2 |
| 8 mm | L-11 | – |
| 9 mm | M/N-13 | – |
| 10 mm | N/P-15 | – |
| 11.50 mm | P-16 | – |
| 12 mm | – | – |
| 15 mm | P/Q | – |
| 15.75 mm | Q | – |
| 16 mm | Q | – |
| 19 mm | S | – |
| 25 mm | T/U/X | – |
| 30 mm | T/X | – |