Hat Size Calculator

Hat size is based on head circumference. US/UK sizes represent approximate head diameter in fractions of an inch; EU sizes equal head circumference in centimetres

The calculator uses the standard published formula for hat size. Results are accurate to the precision of the inputs you provide. For financial, medical, or legal decisions, always verify with a qualified professional.

This calculator works with centimetres, inches. You can enter values in the units shown — the calculator handles all conversions internally.

The core formula is: EU size = circumference in cm. Each step in the calculation is shown so you can verify the result manually.