So you want to pop the question, and you've picked out the perfect ring. But how do you measure her engagement ring size so that the ring will fit perfectly when she says ‘yes'? Short of dragging her into the jeweler and giving away your plans, there are a few others way you can measure engagement ring size that won't give away your intentions to propose.
Get the size of a different ring on her finger. If your soon-to-be fiancée wears a ring on her engagement finger, you can easily measure her engagement ring size by taking note of the size of that ring. You can slyly slip this ring onto your own finger and measure how it fits on your finger. Or you can trace the shape of the ring onto a piece of paper when she's not looking so that you can later match it up to an engagement ring sizing chart.
Slip a ring onto her finger. You can also measure your girlfriend's engagement ring size by putting a different ring on her finger nonchalantly. Maybe you wear a ring on your hand. You could easily slip this off and play with the idea of slipping it onto her ring finger. You could even slip it onto her other ring finger so that your efforts to find her ring size won't be so obvious. And you don't even need to use a ring! Anything round that you can put around her finger that will give you a good idea of her engagement ring size will do the trick. Buy her a costume ring from a vending machine and squeeze it onto her finger. Slip the plastic ring from your pop bottle onto her ring finger and tell her it's all you could afford. She'll laugh at your joke, and not realize that you just measured her engagement ring size.
Tie something around her finger. You can easily measure the circumference of your girlfriend's finger by using a piece of string or paper. Anything you can wrap around her finger will do. Note the length, and then find a Sears catalogue. Flip to the ring section, and place the length of string against the ring size chart provided in the magazine. You'll have discovered her engagement ring size without having to use a ring at all.
Visit a jeweler. If your girlfriend knows what you're up to, you may just want to skip the stress and get her finger sized at the jeweler's. There they have sample rings to try on of various sizes, as well as a sizing chart and other tools to ensure that her engagement ring is the perfect fit.