How to find her ring size without her knowing
The most reliable secret method is to borrow a ring she already wears on that finger and measure its inner diameter. 17.3 mm is a US 7. If borrowing is impossible, ask her closest friend, or order a US 6 to 6½, the most common women’s size, and budget for a resize.
You want the proposal to be a surprise, which rules out the one thing that would settle the question immediately. Here are eight methods that do not involve asking, ranked by how accurate they are and how likely they are to give you away.
Which secret methods actually work?
| # | Method | Accuracy | Risk of being caught |
|---|---|---|---|
| 1 | Borrow a ring and measure it | Very high | Low |
| 2 | Photograph a ring next to a coin | High | Very low |
| 3 | Ask her closest friend or sister | High | Medium |
| 4 | Get her existing ring sized for cleaning | Very high | Medium |
| 5 | The sleeping-hand string trick | Low | High |
| 6 | Buy a costume ring and watch it fit | Medium | Medium |
| 7 | Compare her hands with someone whose size you know | Low | Very low |
| 8 | Order the average and plan a resize | Low | None |
The top three are worth your time. Numbers five and seven are worth skipping entirely, for reasons covered below.
How do you borrow and measure a ring?
This is the best method available, and it is the only one that measures the actual target rather than inferring it from something else.
Find a ring she already wears on the ring finger of the correct hand and measure its inner diameter. Two details decide whether this works.
Take it from a jewellery box rather than from her hand. Rings that live in a box are not missed for an hour. Rings she wears daily are missed in ten minutes, and the one you most want to measure is usually the one you cannot take.
Note which finger it came from. A ring from her right middle finger tells you very little about her left ring finger. If the only ring you can borrow is from the wrong finger, treat the number as a starting point rather than an answer, because most people’s ring finger runs half a size to a full size smaller than the middle finger on the same hand.
Measuring takes under a minute on a calibrated screen. Put the ring on the display, match the circle to the inner edge, read the millimetres, put the ring back.
Can you photograph the ring instead?
Yes, and it is a genuinely good fallback if you cannot keep the ring long enough to measure it properly, or you are somewhere you cannot get your phone out twice without it looking odd.
Put the ring flat on a table next to a coin or a credit card and take one photograph from directly above. Both are precise references: a US quarter is 24.26 mm across, a UK pound coin is 23.43 mm, and every credit card on earth is 85.60 mm wide under ISO/IEC 7810. From the photograph you can scale the ring against the reference later, at your own pace, on your own phone.
Shoot from straight overhead rather than at an angle. A tilted photograph distorts the circle into an ellipse and reads small, which sends you half a size in the wrong direction. Rest the phone flat above the table if you can, and take three shots so you can discard any that are not square on.
Who should you ask?
Less glamorous than the tricks, and more effective than most of them. Somebody in her life already knows her size, or can find it out in a way that raises no suspicion at all.
Choose the person who can keep a secret over the person who is closest to her. Those are not always the same person, and the number of proposals spoiled by an excited sibling is not small. Ranked by reliability: a close friend who has known about your intentions for a while, then her sister if you know her well, then her mother if you know her well and she is not a talker.
The script that works is boring and specific. Tell them you are going to propose, that you need her ring size, that you do not want her to know, and ask whether they can find out without asking her directly. Vagueness invites improvisation, and improvisation is what spoils surprises.
What about sending a ring for cleaning?
If you can plausibly offer to get a ring cleaned or a loose stone checked, a jeweller will size it exactly while it is in the shop. This gets you a professional measurement with a built-in cover story, which is a rare combination.
The cost is that she notices the ring is gone for several days, so it works best with something she wears occasionally rather than daily. It also requires the offer to sound natural, which depends entirely on whether you are the sort of person who has ever previously offered to get anything cleaned.
Which methods should you avoid?
Two, and both appear in almost every listicle on the subject.
The string-while-she-sleeps trick is the worst option on this page. You will wake her, you will pull the string to a tension that means nothing, and a sleeping hand is at its smallest of the entire day. Even a perfect measurement taken that way reads half a size small, and an imperfect one reads worse. Skip it.
Comparing her hands against a friend or relative whose size you know is almost as bad. Hand size correlates loosely with finger size and not at all with knuckle shape, which means the comparison tells you very little. Use it as a rough sanity check on a number you already have, never as the number itself.
Does the costume ring trick work?
Reasonably well, and it is the best of the remaining options.
Buy an inexpensive ring in a size you suspect is right, give it as a small unrelated gift, and watch what happens. If it fits her ring finger, you have your answer. If it is loose or tight, you have a direction and a rough sense of how far out you are.
The tell is what you say when you give it. “I saw this and thought of you” survives scrutiny. “Try this on your left ring finger” does not, and neither does looking too interested in the result.
What should you order if nothing works?
Order the most common size and plan for a resize. In the US that is a 6 to 6½ for women, and a 9 to 10 for men.
Before you buy, get three things in writing. That one free resize is included, and within what window, since many jewellers offer this on engagement rings but do not advertise it. Which direction they can resize, because sizing down is cheaper and structurally safer than sizing up. And whether the design can be resized at all, since full eternity bands, tension settings and channel-set rings often cannot without remaking the piece.
A ring that is a little loose can be worn to the proposal and resized afterwards. A ring that does not go past the knuckle cannot be put on at all, which is a photograph you only get one chance at.
What else will trip you up?
Two things that no amount of accurate measuring will solve on its own.
Her knuckle. If she has slim fingers and prominent knuckles, the ring has to clear the knuckle and then sit at the base without spinning. That is a fitting problem rather than a sizing problem, and it is worth reading the big knuckles guide before you order anything, because the fix is sizing beads rather than a different number.
The band width. If you are buying something chunky, add a half size. A 6 mm band at a US 6 feels closer to a US 5½ on the same finger, for reasons the wide band guide explains.
Can you check the style as well as the size?
You can, and it costs you nothing beyond a photograph you probably already have.
If you have a picture of her hand, Ring Sizer’s AI try-on will render a ring from the gallery onto it. That is a low-risk way to check whether a particular setting suits her fingers, long, short, slim or broad, before the box is closed and the speech is written.
It also has a practical advantage over browsing: it happens inside an app rather than leaving a search history full of engagement rings on a shared laptop, which is how a surprising number of surprises come apart.