Usr measuring tape
Use a tape measure. Wrap the tape measure around the wrist on which you plan to wear your watch bands.
Don't have a measuring tape?
Use a piece of string to measure your wrist and lay the string back out on a ruler to get your measurement in inches or centimeters.
Download and print the Aerial measure tape. Before printing, make sure the "page scaling" option is set to "None" in the dialog box of your printer options. The Aerial measure tape must be printed on full-size standard letter paper. Follow the instructions and make a note of your wrist size.