Do not ask her mother.
We lead with that because it's the most common instinct and one of the leakiest. Mothers are thrilled. Mothers call people. If you need an accomplice, pick someone whose loyalty runs to you and who has no group chat stake in the outcome.
With that out of the way, here are the methods that actually work, roughly in order of how reliable they are.
Borrow a ring she already wears
This is the gold standard, and the only thing that gets you a real measurement instead of an estimate.
Take a ring she wears on her left ring finger — that qualifier matters, because fingers differ by up to a full size across a hand, and a ring from her right middle finger will send you wrong. Bring it to any jeweler and they'll size it in thirty seconds, free.
Can't get out to a shop? Set the ring on a printed ring-size chart, or trace the inside circle on paper and measure the diameter in millimeters. A 16.5mm inner diameter is about a US 6; 17.3mm is about a 7. Rough, but usable.
Return it before she notices. That last step is where people get caught — take it from a jewelry box she doesn't open daily, not off the dish by the sink.
The string trick, and how wrong it can go
You'll find advice to wrap string or floss around her finger while she's asleep. It technically works. It's also the least accurate method on this list, and worth being honest about.
String stretches. Pull it snug and you'll undershoot by a half size or more; leave it loose and you overshoot. And you have to get it past the knuckle, which is what actually determines whether a ring fits — plenty of people have a slim finger and a wide knuckle, and the knuckle wins.
Use it as a sanity check on a guess, not as your only data point. If it's the only thing you've got, round up.
Ask the right friend, not the obvious one
The right accomplice has three qualities: she knows her jewelry, she's structurally incapable of gossip, and she can bring it up casually. Often that's not the best friend — the best friend is the first person she'll interrogate when she gets suspicious.
Good cover stories, all of which have worked:
- Rings-as-a-topic while shopping together — cheap costume rings, tried on for fun, sizes noted.
- A "gift for my sister" question that gets her comparing hand sizes with someone.
- A birthday ring you actually give her, months earlier, from which you learn everything.
That last one is slow but nearly foolproof, and you get credit for a present along the way.
When to just size up
If you've narrowed it to two sizes and can't resolve it, take the larger one. A slightly loose ring stays on her finger during the proposal and gets sized down cleanly afterward. A ring that won't pass the knuckle turns the moment into a wrestling match, and she'll be the one apologizing for it.
For reference, the most common women's size in the U.S. is a 6 to 6.5. If you have genuinely nothing to go on, a 6.5 is the least-bad blind guess. Also worth knowing: fingers swell in heat and after salt, and shrink in cold, by up to a half size. If she's petite, lean 5 to 5.5; if she's tall or works with her hands, lean 7.
Resizing: what's easy, what isn't
Most rings resize a size or two either direction in about a week for $40 to $80. Plain gold and platinum bands are simple work.
What's harder or impossible:
- Eternity bands with stones all the way around — there's no plain metal to cut, so resizing means rebuilding.
- Full pavé shanks — doable, but the jeweler has to reset stones, so it costs more and takes longer.
- Tension settings, where the metal's spring holds the stone. Changing the diameter changes the tension. Usually a no.
Which means: if you're guessing on size, guess on a solitaire or a partial-pavé design rather than an eternity band. That single choice keeps your options open. Our rundown of setting styles flags which ones resize easily.
Check the return and resizing terms before you buy, too — ours are in the sales policy, and any seller worth using publishes theirs plainly.
One last thing. If the size ends up wrong on the night, it does not matter even slightly. Nobody remembers that the ring was loose. They remember what you said.