Use soft measuring tape to measure the circumference of your head about 1 cm above your eyebrows. Make sure the measuring tape is at the same height all around your head when measuring.
If you do not have a soft measuring tape, usa a string in the same way indicated above to measure your head. Then lay the string straight and measure it with ruler.
You can use inches but centimeters are more precise. Use our size chart to guide you the correct letter size.