Best thing would be to go to a jewelry shop or studio and check your finger or a ring , but, if you really wanna do it yourself , check out the D.I.Y. links below :
BRACELET SIZE GUIDE
How to determine your bracelet size:
Wrist measurement + 1 inch
Wrap a measurement tape around your wrist (just below the wrist bone), not too loose or tight. Add one to one-half inch to the measurement to ensure a comfortable fit. For example, if your wrist measures 6 inches, a bracelet of 7 inches will fit your wrist comfortably.
No measurement tape handy? Wrap a piece of ribbon or yarn around the wrist (below the wrist bone) snugly. Measure the ribbon or yarn with a ruler to get the length measurement. Add one to one-half inch to the measurement to ensure a comfortable fit.
Use this to convert between standard size formats.