How To Find Her Ring Size
So you did some investigation. You've gone through her jewelry box to see what style she likes, paid attention to what she wears everyday and on a big night out, subtly asked about that nickel allergy and you did it. You found the ring she is going to say yes to.
But how do you know what size she is? Here are a few ways to find out while still keeping it a secret. Let's start with the average woman that is 5'6" and 140lbs., and a ring size of 6. You can adjust up or down depending on how slender her fingers are, or you can be sneaky and do some investigating. If you can find a ring she wears on the ring finger of her right hand that will be the best measurement (fingers on the dominant hand are usually half a size bigger), but if you can get one she wears on her middle finger you can just take it down a size and that should be close.
Do you have access to her jewelry box? Does she take her rings off to sleep or in the shower? Find a ring she wears, and trace around it on a piece of paper. You can also slide it on your finger and use a pen to draw two lines where it stops. A jeweler or a ring sizer online (make sure it prints at 100%) will help determine what size it is.
You can also press the ring into a bar of soap to make an impression. This works best with rings that do not have filigree or set stones that will get all full of soap, but if that happens, rinse the ring carefully in warm water to wash the soap away.
Is she a heavy sleeper? Measure around her ring finger while she is asleep with a piece of string (don't pull it tight) or paper.
Get Her Talking
Other sneaky ways to find that measurement: choose one that's most in character for you.
Planning way ahead? Get her a promise ring in a large size. Take it together to a jeweler to get it resized, and they will measure her for a good fit. As you are already in the store looking around, this is also a good way to get her thoughts on engagement ring settings and styles.
In the guise of choosing a ring for your mom or sister, ask her help and have her try on some rings. You can always decide at the end of the day to get your mom a necklace.
If you are attending a street market or craft fair anytime soon, there are usually many vendors selling rings, and the sizes are often displayed. Pick out one in size 6 that is not her taste, and ask her to give you her hand. You can tease that it's her next birthday gift! If she refuses, pick out something better and she may actually allow you to try one on her. If you can learn the size of any of her fingers this will help.
Tell her you learned an amazing fact — if you take your shoe size, divide by two and add three, you get your ring size. Have her walk you through this calculation if you can,obviously.
Buy a cheap costume jewelry ring in the size you think she might be, but not so cheap it is open in the back. Tell her your friend bought it for a joke, or you thought you'd dress up as Marilyn Monroe for Halloween, or remove the packaging and say you found it in the street, whatever seems plausible.
Choose someone she is close to, a sister or friend, that could steer her toward a jewelry store (or that booth at the craft fair) when they are shopping, presumably to try on rings for themselves. This also may be useful for gathering information if you have not yet picked out the ring!
Find one of those email surveys that asks about their favorites, the last song they listened to, whether they like mountains or the beach, etc., and insert the ring size question in there. Get a mutual friend to fill it out and send it to her and a few friends, or if it is not out of character, do it yourself.