We recommend the below:

• Put one of your fiancé-to-be’s rings on your finger and mark with a pen how far down it goes. Then, head to the jewelry store and use their ring sizer to determine which ring size hits the same spot.
• Alternatively, you can download and print this easy to use ring sizer. 
• If you and your fiancé have discussed your future marriage plans, you could simply ask what his or her ring size is.
• Keep in mind: Rings worn on the ring finger of the non-dominant hand should be about ¼ size smaller than rings worn on the ring finger of the dominant hand.