How to Figure Out Your Girlfriend's Ring Size Make your proposal more surprising by figuring out her ring size in advance By Yu Qing Popping the question while slipping a ring that fits perfectly onto your new fiancee's finger makes for one romantic proposal. Figure our your girlfriend's ring size in advance to make sure your proposal goes smoothly. Your fiancee will want to be showing off her new engagement ring to her friends and family, and she'll be able to do so right away because no resizing was needed. Become a Super Sleuth If your girlfriend wears a ring on her ring finger, look for an opportunity to borrow it when she isn't wearing it. Take it with you to a jeweler and they can help you determine her exact size. Trace the Ring Trace the inside diameter of the ring on a piece of paper. This is a quick way to measure her ring size if you're afraid she would notice her missing jewelry if you took it to a jeweler. Create a Mold Make an imprint of her ring in soap. This can be done if your girlfriend takes off her ring when she's in the shower. Press her ring into a piece of soap, then bring the imprinted soap to a jeweler and figure out which size fits the mold. Be Your Own Hand Model Use your own finger. This works well if you're running out of time and lack other measuring tools. Slip her ring