Planning to surprise your girlfriend with a ring—whether it’s an engagement ring, a promise ring, or a “just because” gift—is one of the most exciting (and nerve-wracking!) moments. The last thing you want is to ruin the surprise by asking her size directly… or worse, end up with a ring that doesn’t fit. Luckily, you don’t need to be a detective to figure out her ring size secretly. These 5 tricks are simple, low-risk, and let you keep the surprise intact—all while making sure the ring fits like it was made just for her.
If your girlfriend owns any rings that fit her ring finger (or even a similar-sized finger, like her middle or index), this is your best bet. Wait for a moment when she’s not wearing it—like when she’s in the shower, sleeping, or doing the dishes—and grab it quickly.
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What to do next: Trace the inside of the ring on a piece of paper (make sure the trace is clear, not smudged). Or, place the ring on a ruler and measure the inner diameter (from one edge of the inside to the other) in millimeters. Most jewelers can use this measurement to find her size instantly.
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Pro tip: If the ring is thin (less than 2mm wide), add 0.5mm to the diameter—thin rings often feel looser than wider ones.
This trick works when you’re together—like watching a movie, cuddling on the couch, or even cooking together. Grab a piece of thin string (or a strip of paper, 1cm wide) and keep it in your pocket.
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How to do it: Casually wrap the string around her ring finger when she’s not paying attention—say, while you’re holding hands, or when she’s resting her hand on your lap. Mark the spot where the string overlaps with a pen (use a tiny mark so she doesn’t see it). Later, measure the length of the string from the start to the mark in millimeters—that’s her finger circumference.
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Pro tip: Do this 2-3 times to get an average—fingers can swell a little depending on the time of day (they’re usually smaller in the morning!).
Her closest friends or siblings probably know her ring size—either she’s mentioned it before, or they’ve shopped for jewelry together. This is a great option if you’re worried about messing up the “borrow” or “string” tricks.
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How to ask: Send a quick, private message (avoid group chats!) saying something like: “I’m planning a little surprise for [Her Name] with a ring—do you happen to know her ring size? I don’t want to ruin the surprise by asking her directly!”
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Pro tip: If they don’t know the exact size, ask if she’s ever mentioned a “US size” (common in the US) or “EU size” (common in Europe)—jewelers can convert these easily.
Pretend you’re curious about jewelry for yourself (or for a “friend”) and suggest stopping by a jewelry store during a date. This lets you get a professional measurement without her suspecting a thing.
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How to play it cool: Say, “Hey, let’s pop into this jewelry store—my cousin needs help picking a ring, and I have no idea how sizes work. Can you help me test a few to see how they fit?” Then, hand her a few rings to try on (start with simple bands, not anything too fancy). The jeweler will likely note her size when she tries them on—you can ask the jeweler later for the number.
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Pro tip: Pick a store with friendly staff—most will be happy to help you keep the surprise if you mention it quietly.
If you have a smartphone, there are free apps (like “Ring Sizer” or “Ringsize”) that let you measure a ring’s size using your camera. This works best if you can “borrow” one of her rings (like in Trick 1) for 2 minutes.
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How to use it: Place the ring on a flat surface (like a table) and use the app to take a photo of it. The app will measure the inner diameter and tell you her size. Make sure the ring is straight in the photo—tilted rings can give wrong measurements.
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Pro tip: Use a white background (like a piece of paper) behind the ring to help the app focus better.
If you’re still unsure—say, the measurements are between two sizes—choose the larger one. Most jewelers can resize a ring to make it smaller (it’s harder to make a ring bigger without changing its design). And if the ring is a little loose at first? You can add a tiny silicone ring guard (invisible from the outside) until you get it resized.
At the end of the day, she’ll care more about the thought behind the ring than if it’s perfectly sized on day one. But with these tricks, you’ll get pretty close—and keep the surprise alive until the moment you give it to her.
Ready to start planning? Your secret’s safe with us.
