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Crochet Hook Size Conversion
US letter and number crochet hook sizes converted to metric millimetres. Metric is the international standard.
| US | Metric (mm) |
|---|
Standard: ISO 2560 (metric sizes) · Data checked: 2026-10-14
Frequently asked questions
Why do hook sizes differ between US and UK?
The two systems grew separately and the UK historically used a numbering that ran opposite to the US. As with knitting needles, the millimetre size is the only figure that means the same thing in both.
How do I match a hook to a yarn?
Start with the hook on the ball band and adjust to hit the gauge the pattern specifies. Hook choice also depends on the stitch pattern: taller stitches generally want a slightly larger hook than the yarn's nominal size.
What is the difference between a steel and an aluminium hook?
Steel hooks are made in the small sizes used for thread crochet, typically 0.6 to 2.25 mm, and are labelled differently again from the aluminium and plastic hooks used for yarn. Make sure you are reading the right scale for the material.
What does the letter on a hook mean?
It is the US designation, and it is not consistent across manufacturers. A hook labelled I may be 5.5 mm from one maker and a slightly different figure from another, which is why the millimetre size is worth more than the letter.