The reliable methods
- Borrow a ring she wears on that finger. Take it to any jeweler for two minutes, or trace the inside circle on paper several times and measure the diameter. Return it before she notices. This is the gold standard.
- Recruit an accomplice. A friend or sister can get her ring size in one casual shopping trip. Pick someone who can keep a secret for more than a week.
- The compare trick. Have her try on someone else's ring "for fun" and note where it fits, or slip one of her rings onto your own finger and mark how far it goes with a pen.
If you are flying blind
The most common women's sizes sit around 6 to 6.5. If you truly cannot measure, err a half size up. A slightly loose ring goes on for the proposal photo. A ring that will not pass the knuckle does not.
The safety net
Most rings resize up or down a size without drama, and reputable sellers include or discount the first resize. The exceptions: full-pavé bands and tension settings are hard to resize, so get those closer to right the first time.
Translation: do not let size anxiety delay the proposal. Close counts.
Bottom line
Trace a borrowed ring if you can, round up if you cannot, and buy from somewhere with a friendly resizing policy. This part is very survivable.