To get the correct size for a Tilley hat, Tilley recommends measuring the head with a tape measure. The bottom of the tape measure should touch just above the eyebrows in a straight line around the head. Prospective buyers can then use this measurement to consult the Tilley sizing chart.
Tilley designs its hats to be slightly loose and to be worn low on the head. A trick to determine if the Tilley hat fits correctly is to insert two fingers between the middle of the forehead and the front of the hat, and then rotate the hat left, right, up and down. If the wearer is able to do this without trouble, the Tilley hat fits properly. If not, the wearer should change the size.