Finding one's hat size is a process of taking accurate measurements, considering the kind of hat that the measurements are for, and considering where the hat will be made. As with many other types of clothing and accessories, sizing charts vary from country to country. A hat size in the United States does not necessarily correspond to a hat size in the United Kingdom, even if the sizes are numerically identical. The best way to determine one's hat size is to find a sizing chart and sizing instructions distributed by a hat company that one plans to patronize. This is the best way to take accurate hat size as the information should also include specific instructions about how and where to measure the head.
For those who are interested in learning their hat size for more general purposes, there are basic measurements that can be taken. In order to take these measurements one will need a tape measure made of cloth or flexible plastic and, perhaps, a friend to help take the measurements. The circumference of the head should be measured at the widest part of the head. First, take the tape measure and measure 1 inch (about 2.5 cm) straight up from the arch of the eyebrows. This will determine the latitude on the head from which the circumference can be taken.
Have a friend measure this circumference two times. If the measurements differ, try a few more times until a dominant measurement presents itself. This is most likely the accurate measurement. In general, a circumference that is 21 inches (about 53 cm) is considered to be small. A person with this hat size will wear hats that are sizes small or extra small.
A circumference between 21 inches (about 53 cm) and 22 inches (about 56 cm) would also be considered small, but not extra small. A medium sized hat would fit a person with a head circumference between 22 inches (about 56 cm) and 22.5 inches (57 cm). A person with a head circumference between 22.5 inches (57 cm) and 23.5 inches (60 cm) would probably wear a large-sized hat. Those with head circumferences that exceed these measurements would have to wear hats that are sized as extra large and higher. Those who have especially large heads may have to have their hats custom made whereas those with more common hat sizes can usually purchase hats that have already been fabricated.