Crochet hook sizes use the metric system to measure the width, sometimes called the gauge, of the hook. The many sizes of crochet hooks can range from less than 1 millimeter to 20 or more. Hook size is extremely important to a crochet project, determining the gauge of the stitches and the finished size and structure of the item. When crocheting from a pattern, make sure to use the recommended crochet hook size and yarn thickness to ensure that your completed product turns out the correct size, shape, and texture.
In addition to the metric system measurement, crochet hook sizes are usually accompanied by a letter or another number that non-metric system users can use to find the size of the hook. For example, in the United States, a hook may be called a size "J" or a size ten, both of which are names for a hook that is 6 millimeters in diameter. Also, very tiny hooks, usually used for making lace or crocheting with fine yarn or thread, have U.S. sizes that range from the largest at "00," or 3.5 mm, down to a miniscule .75 mm, called a size 14. These tiny hooks do not have a letter size associated with them, unlike the larger crochet hook sizes. Less commonly, U.K. and Canadian sizes, different from both the regular metric measurement and the letter and number combination of the U.S. system, are also included.
Depending on the size of the crochet hook, it may be made out of plastic, metal, wood, or even sometimes bone. Tiny hooks tend to be made out of steel for durability, while very large hooks are generally plastic or another lightweight material. Middle-sized hooks are frequently aluminum, but can be made out of any of the other materials as well.
Crochet hook sizes can greatly alter the way a finished project looks and feels. Using a large crochet hook will give the project a more lacy, open feeling because of the space left between and throughout the stitches to accommodate the large hook size. Conversely, small hooks will result in a tighter, closer stitch formation and smaller stitches overall. Most patterns include a gauge at the beginning, stating exactly how much area a swatch of a certain number of stitches should take up in order to make sure the finished piece ends up the right size.