Crochet hook sizes are determined by the diameter of the shaft. This can be measured with a crochet gauge which also tells you the correct letter and numeric classification of the hook.
To properly measure the crochet hook, it is necessary to locate the shaft. This is the part of the hook immediately south of the narrow portion of the tip. Place the hook into the series of holes on the gauge. If the hook is too large for the specific hole or if it is too loosely set, continue searching for the correct hole. If a gauge is unavailable, this measurement can also be taken with a tape measure. Jumbo needles may be too large to fit in an average gauge.
Jumbo crochet hooks come in a variety of different sized diameters including: 12 millimeter, 15 millimeter, 20 millimeter, 25 millimeter and 30 millimeter. These hooks are ideal for working with extremely bulky yarns. Hooks with these diameter lengths also come in a variety of lengths ranging from 7.5 inches to 15 inches.
Many modern needles come with the diameter of the shaft stamped into the hook itself. These hooks are also likely to have a corresponding letter categorization.