You need a measurement from someone who must not know you are taking one. Some of the popular methods work; several do not.
If you manage to borrow a ring, measure it and enter the diameter here.
Your size
You need a finger measurement from someone who must not know you are taking one. The methods below are ranked by how often they actually work, because several of the popular suggestions do not.
If she owns a ring worn on the target finger, this method has a clear advantage over the alternatives below: you are measuring a rigid object with a fixed size rather than estimating from a person.
Take it while it is off — overnight, in the shower, at the gym — measure the inside diameter, and put it back before it is missed. You need it for a few minutes, not a few days. How to measure a ring's inside diameter
It has to be a ring from the same finger on the same hand. A ring from her right middle finger tells you very little about her left ring finger — different fingers vary considerably, and the same finger on opposite hands commonly differs by around half a size. If you are unsure which finger a ring came from, note where it was sitting before you take it.
Press it into a bar of soap, a stick of lip balm, or a piece of plasticine to leave an impression, then measure the impression. Or lay it on paper and trace the inside circle carefully with a sharp pencil, holding the pencil vertical. Both are less accurate than measuring it directly, but both work.
A close friend, a sister, her mother. People underestimate how effective this is, usually because they overestimate how suspicious it sounds. A friend can ask directly in conversation without it registering as unusual.
A friend can also simply try a ring on for size and report back, which converts the problem into an ordinary measurement.
If you are shopping together for anything else, a jeweller will size her hand as part of normal browsing. This is entirely routine and jewellers are practised at it.
A version that works well: buy her a fashion ring as an ordinary gift and have it properly sized in the shop. You get a professional measurement and she gets a present, and nothing about it looks like groundwork.
This one appears in every list and is mostly folklore. Fingers are soft, so a string or paper wrap on a sleeping hand compresses the flesh unpredictably, and you cannot check knuckle clearance without waking her. Expect to be a size out in either direction.
If you try it, do not pull tight, take three readings, and treat the result as a rough bracket rather than an answer.
Hold her hand and note where her ring finger reaches against yours. If you know your own size, this tells you roughly how far off hers is. It will not give you a size, but it will tell you whether you are looking at a US 5 or a US 8, which is useful when every other method has failed.
US 6 and 7 are the most commonly ordered women's sizes; US 9 and 10 for men. Those are the busiest part of a wide distribution, not a prediction about a specific person, and finger size correlates only loosely with height or build.
If you are down to this, size up rather than down and plan on resizing.
If the ring is for a child rather than an adult, the sizing works differently and the chart runs below US 3 — see children's ring sizes.
Sizing a surprise is difficult by design. Several ways to sidestep it entirely:
Sizing up is only a safe fallback if the ring can be altered. A full eternity band, a tension setting, or anything in tungsten, ceramic or titanium may not be resizable at all. Confirm before you buy — this should influence which ring you choose, not just which size. Which settings can be resized
Go up. A ring that will not pass her knuckle cannot be put on in the moment, which is the one outcome worth avoiding. A slightly loose ring goes on, looks right, and can be sized down in a fortnight.
Bear in mind too that the moment itself is often warm hands, adrenaline, and possibly a summer evening — all of which push in the direction of a larger finger, not a smaller one. Why fingers swell
Ring sizing figures get copied between websites without anyone verifying them, so an error introduced once spreads quietly. Rather than ask you to take these numbers on faith, here is what stands behind them — each of which you can check.
Everything here is a close estimate rather than a guaranteed fit, and published charts differ between manufacturers by up to roughly a quarter of a size. Confirm the size against the specific retailer's chart, check whether the ring can be resized at all, and read the returns policy before ordering.
Borrowing a ring she already wears on the target finger is the most reliable of these methods — measure the inside diameter and return it within minutes. Failing that, ask a close friend or family member, or have a jeweller size her during ordinary browsing.
Rarely well. Fingers are soft, so wrapping a sleeping hand compresses the flesh by an unpredictable amount, and you cannot test knuckle clearance without waking her. Treat any result as a rough bracket that could easily be a size out in either direction.
Size up. A ring that will not clear her knuckle cannot go on in the moment, whereas a slightly loose one goes on fine and can be sized down later. US 6 to 7 is the most commonly ordered women's range if you are guessing blind, but confirm the ring can actually be resized first.
Yes. Press it into a bar of soap or a stick of lip balm to leave a clean impression and measure that, or lay it on paper and trace the inside circle with a sharp pencil held vertically. Both are less precise than measuring the ring directly but perfectly usable.
It is an increasingly common approach and it removes the sizing problem entirely. It also lets her have a say in a ring she will wear for decades. It is particularly worth considering if you are drawn to a full eternity band or tension setting, since those often cannot be resized afterwards.
How to read the inside diameter accurately in the few minutes you have.
Can the ring be resized?Check this before you rely on sizing up as your fallback.
Typical women's sizesWhat the averages are, and why they are a weak guide for one person.
Choosing between two sizesThe reasoning behind sizing up, and its exceptions.