It is easier to get someone to take your head measurement than to do it yourself. However, this is how it is done.
Take a tape measure and place one end in the centre of your forehead at around the level where you would like your hat to sit, usually between one and two centimetres above the eye brows. Pass it around your head and back to the middle of your forehead. If there are any bumps make sure these are included.
This should read between 55 and 59 centimetres for most women, and between 56 and 62 centimetres for most men. Add a half centimetre to the reading unless you want the hat to be very tight.
Remember, if it is too tight it may leave a red mark on your forehead.