Work out hat size converter instantly with clear inputs, formula shown and shareable results.
Traditional hat sizes are the head diameter in inches, obtained by dividing the circumference by pi. A 57 cm head is 22.44 inches around, giving a UK size of about 7.14 - normally sold as 7 1/8. US sizes run an eighth larger than UK for the same head, and letter sizing bands every 2 cm.
Hat size
size = circumference(in) / pi
US offset
US size = UK size + 1/8
Around the head about a centimetre above the ears and eyebrows, at the widest point, with the tape snug but not tight.
Because they descend from diameter in inches, and an eighth of an inch is about 1 cm of circumference - the practical resolution of a fitted hat.