Use a paper measuring tape to measure around your wrist. if you do not have one, you can use a piece of string to wrap around your wrist & then measure the length with a regular ruler. 

Do not pull the paper measuring tape or the string too tightly.

Inches	Centimeters
6.5 in	16.5 cm
6.75 in	17 cm
7 in	17.8 cm
7.25 in	18.4 cm
7.5 in	19 cm
7.75 in	19.6 cm
8 in	20 cm
8.25 in	20.9 cm
8.5 in	21.6 cm
9 in	22.86 cm