If you are absolutely set on a surprise proposal but you still want to find out her ring size directly from the source, ask your girlfriend in a very subtle, smooth way. You can pretend, for example, that you are going to buy a ring for your mom/sister/aunt/niece etc. Make sure that this pretend recipient is someone whose hands are similar in size to your girlfriend. Casually ask what her ring size is for reference. If you really want to beef up the ruse, purchase an inexpensive ring and show it to her. Have her try it on and everything! Maybe afterward you can actually give that ring to your mom/sister/aunt/niece to appease your guilty conscience!
Ask Her Friends
A great way to find out her ring size is to ask one of her friends. Friends occasionally have this insight! And, if the friend doesn’t know right away, she can be the one to ask (nonchalantly). Even better, ask the friend to take your girlfriend engagement ring shopping “for fun.” Have her get professionally sized and ask her friend to report back to you. It’s best to choose a friend who might have once had acting aspirations!
Borrow One of her Rings
Another method is to snag one of her old rings for a day. You need to be careful with this method because it might backfire on you! Make sure that you grab a ring that she wears on a similarly sized finger, the ring finger of her right hand being the optimal choice (unless, of course, she wears a ring on her left ring finger, which is less likely). Even if you bring in a thumb ring we can usually come up with a close approximation of what size her ring finger will be. If you choose this option, make sure that you choose a ring that isn’t overly important to her and wouldn’t cause any emotional distress if she notices it missing!
Leave a Trace
If you are terrified of losing one of her rings try this method instead. Take one of her rings and place it on a piece of paper. Draw a circle on the inside and outside of the ring. Do this several times to make sure that your ring traces look consistent. We can usually figure out the diameter of her ring from your sketch!
Enlist a Friend
If one of your partner’s friends is engaged, ask her friend to let your girlfriend try on the ring. This will put you in a ballpark size range to start out with. You can also use this as an opportunity to ask what she thought of her friend’s ring (in private) and try to do a little sleuthing as to what style of ring she would like for herself.
This method is a little more difficult, but if you are desperate, go ahead and try it out! When you are holding her hand,see if one of her fingers is similar in size to a finger of yours. For example, her ring finger might be as big as your pinky. This method isn’t amazingly accurate, but the more information we have on her ring size the closer we can get!