Convert US crochet hook letter sizes to millimetres.
Crochet hooks are sold in US letter sizes, metric sizes in millimeters, and sometimes old UK numbers, and the three systems do not line up neatly. The millimeter size is the most reliable, since it is the actual diameter of the shaft. This calculator converts the US size to millimeters, so you can use a pattern from another country.
For example, a US G hook is 4.0 millimeters, an H is 5.0, an I is 5.5, and a J is 6.0. Brands vary slightly, so measure the hook with a gauge if you are unsure, and when a hook is not marked, a hook gauge tells you its size.
The hook should suit the yarn. Check the ball band for the suggested size, and use it as a starting point. A smaller hook gives a tighter, denser fabric and a larger hook gives a looser, more open one, so the size also changes the drape of the finished piece.
Always make a gauge swatch, since your tension is your own. If your stitches are tighter than the pattern, go up a hook size, and if they are looser, go down. The material of the hook, metal, wood, or plastic, also changes how the yarn slides.