Make a print
When your partner is getting dressed; take a piece of soap and make a print. These prints can be measured as an exact size of the finger.
Draw a ring
Take a pencil and put a ring on the paper. Copy the inner side of the ring. This technique is more exacting than the first one and you have to be precise.
Put a ring on your finger
Simply put your partner’s ring on one of your fingers and mark the place where it gets.
Lend it
If you are sure that your partner will not miss the ring, take it and go to the nearest jeweler. He will certainly tell you the size of your partner’s finger.
