If you’re shopping for someone else and planning a surprise, the best way to find her ring size is to ask in a subtle, smooth way. You can pretend, for example, that you are going to buy a ring for your mum/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.Another way is to get help from friends and family. They can ask without raising any eyebrows. 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.Alternatively you could borrow one of her rings. 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). You could then measure it with the instructions outlined above.