What does this tool do?
Hat sizes are built on head circumference, and different countries express the same fit in different ways. This converter takes a circumference or a known US or UK size and shows the matching sizes plus the metric measurement.
The Formula
US size = Head circumference (in) ÷ π; UK size = US size − 1/8; metric size = head circumference in cmFrequently Asked Questions
How do I measure my head for a hat?
Wrap a soft tape around the widest part of the head, about 1 cm above the ears and across the back of the skull. Keep the tape level and snug but not tight, then round up to the nearest eighth of an inch.
Why does the same hat size fit differently between brands?
Crown shape, brim stiffness and the stretch of the material all affect fit. Use this chart as a starting point and check the maker's own size guide where possible.