How to Find a Girlfriend's Ring Size. You have decided you love your girlfriend enough to marry her, but don't know her ring size. No problem. There are ways to find out her ring size. Some are more tactful than others, but all end up with you knowing the size of your girlfriend's ring size for the finger of her left hand.
Ask your girlfriend flat out what size ring she wears on her fingers. If you want to be more obvious, ask what size ring is on her left ring finger. Play coy and pretend like it's for something else, but chances are, she'll know what you were asking for.
Be honest and tell her why you want to know her ring size and let her pick out her ring herself.
Quietly creep into her room when she's sleeping, busy or not home and see what size rings she has.
Pull off one of her rings while she sleeps and size it or use a piece of string to get her size, if she's a heavy sleeper.
Talk to her best friend about what size she wears or have her try to find out. It's less obvious if her best friend asks.
Try to get her to try on one of your rings and estimate the size from that. You could also try on one of her rings and estimate the size that way.