Size medium will be the best size choice for approximately 70% of people. Size M: Head circumference 54-60cm (21" - 23.5")
Size L: Head circumference 60-66 cm (23.5" - 26")
Directions for correct sizing:
Place a string or fabric tape measure around your head about 1/8" above your ear, across the middle of your forehead, completely encircling your head. Hold the tape firmly, but not too tightly. If your head measurement falls at the end of size (for example, 60cm), choose the smaller size.