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How to Find Her Ring Size Without Her Knowing (and What to Do if You Can't)

“The surprise matters less than the ring fitting within a week of saying yes, so don't risk a panic over half a size. A good guess, a resizable setting, and a close friend in on the plot is more than enough.”Her take

Which of her rings should you use?

Look for a ring she wears on the same finger you're shopping for. For an engagement ring in the US, that's the left ring finger. If she doesn't wear one there, a ring from her right ring finger is your next best option, but tell the jeweler which finger it came from. The dominant hand can run a little larger, so a ring from her right hand may be slightly big on the left if she's right-handed.

Avoid rings from a different finger or ones she has stopped wearing, since fingers change over time. Thin bands are the best reference. A wide band sits tighter than a thin one of the same size, so she may wear wide rings a little larger, and a wide ring can measure bigger than she'd need in a thin one.

The borrow method: quickest and most accurate

Pick a ring she won't miss for a few hours, such as one in a jewelry box, not one she wears every day. Take it to a jeweler and ask them to measure it on a ring mandrel, the tapered metal tool used for sizing. It usually only takes a few minutes, and many shops will do it even if you aren't buying that day.

Before you take the ring, photograph exactly how it's sitting so you can put it back in the same place. Plan for a day when she's out, and don't wait until the last week. If you're ordering a custom ring, this gives you the time you'll need.

How to measure a ring at home

If you can't get to a jeweler, you can measure a ring at home. It's less accurate, so use it as an estimate. Place the ring on a piece of paper and trace inside the circle with a sharp pencil, then do this again to compare. Measure the widest point of the inside, in millimeters, not including the metal.

Then match it against the US sizes below. Charts differ by about 0.1 mm, so if you land between sizes, go up to the next one. You can also press the ring gently into a bar of soap and bring the impression to a jeweler later, or take a clear photo with a ruler right next to the ring for a rough reading.

• Inside diameter of about 15.7 mm: US size 5
• About 16.5 mm: US size 6
• About 17.3 mm: US size 7
• About 18.1 mm: US size 8
• About 18.9 mm: US size 9

Who to ask, and what to ask them

Her closest friend, sister or mother may already know, or can find out by trying on rings during a casual shopping trip. Pick one person who can keep a secret and ask a specific question: what size do you think she wears, and on which finger? If they go shopping with her, ask them to note the size stamped or tagged on any ring that fits.

A low-key approach also works: mention a friend's new ring or a wedding, then see if she brings up her own size. You could also suggest stopping in a jewelry store for fun around the holidays, where a sizing can happen in a minute. Just be aware that she may figure it out.

What if you still can't find out?

If you only get a moment with one of her rings, slide it onto one of your own fingers and mark where it stops with a pen. A jeweler can then size your finger at that mark. It's rough, but it gets you into a close range. Then buy a size slightly on the larger side, because a ring that's a little loose will still go on her finger at the moment you need it to.

Before you buy, ask the jeweler these three questions: can this ring be resized, how many sizes up or down, and is the first resizing included? Resizing is often simple on a plain gold or platinum band, but it's harder or impossible with eternity bands, some tension settings, pavé that runs around the full band, and very hard metals such as tungsten. Keep the receipt and check the return policy.

Things that make sizing go wrong

Fingers shift with temperature and time of day. They tend to shrink in the cold and swell in heat, or after salty food or a long flight. So her real size is best checked at a normal room temperature, later in the day, once she's in on the secret. A ring that fits but won't slide over a large knuckle may need a slightly larger size or sizing beads.

Remember that a thick band fits tighter than a thin one in the same size. Many jewelers suggest going up about a quarter to a half size for bands about 6 mm wide or more. If you're also choosing the style, it's worth noting what she already wears: band width, metal and whether she likes stones.

Questions people ask

The takeaway

Borrow a thin ring from the right finger, have it measured, round up when you're between sizes, and buy a setting that can be resized.

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