To determine your hat size, measure the circumference of your head. To do this you need a flexible tape measure and a mirror, you can also use a piece of string and a ruler. Place the tape measure or string around your head and wind both sides round to the front, placing it just above your ears and above your eyebrows. Bring the tape measure together in the middle of your forehead. Don’t pull the tape measure too tight! Read the measurement and repeat the process a few times.