Crochet hooks come in a wide variety of sizes, ranging from less than a millimeter in diameter to almost two centimeters thick.
The smallest hooks are made from steel and are used with fine threads to make delicate laces. These hooks are sized 00-14, with 00 being the thickest at 2.70 mm and 14 being the finest at 0.80 mm.
The medium-sized hooks, which are used for the majority of yarn crochet projects, are usually made from aluminum but can also be found in plastic or wooden varieties. They are assigned both a letter size (A-K) and a number size (0-10½), and range in diameter from 2.0 mm to 6.5 mm. The exception to the letter size system is the number 7 hook (4.5 mm), which does not have a corresponding letter.
The largest hooks are plastic and can be used with thick yarns or strips of fabric to make bulkier projects. They are sizes L-S (8.0-19.0 mm), and may or may not have a number size in addition to the letter size.
Although the Craft Yarn Council of America has attempted to standardize the sizing system for crochet hooks, some discrepancies may still be found between the letter and number size systems. To ensure accuracy when following a pattern, it is always best to reference the millimeter size of the hook. An H (5.0 mm) - J (6.0mm) size hook is the best size to use when you're first learning to crochet.