If an engagement ring purchase falls apart for one reason, it’s often this one: “I don’t know her ring size.” That’s a completely normal fear—because choosing the wrong size feels like a big risk.

Good news: you can estimate ring size surprisingly well in secret, and even if it isn’t 100% perfect, there are simple solutions (temporary adjusters, resizing later, etc.). The most important thing is that the ring fits on her finger in that important moment.

Based on Keefirivunts experience, a useful reference point is: the average engagement ring size is usually around 16.5–17.5 mm (inner diameter).

Quick Summary (if you’re in a hurry)

  • If you’re between two sizes: choose slightly bigger.

  • If you can get one of her rings: measure the inner diameter (mm) or trace the inner circle on paper.

  • If you have no idea at all: 16.5–17.5 mm (EU 52–55) is very often a safe range.

  • If after the proposal the size isn’t ideal: a temporary adjuster + resizing later is completely normal.

What does “16.5–17.5 mm” actually mean?

Ring size can be measured in different systems. The easiest is to think in terms of diameter (inner diameter in mm).

Approximate equivalents:

  • 16.5 mm diameter ≈ EU 52

  • 17.3 mm diameter ≈ EU 54

  • 17.5 mm diameter ≈ EU 55

The average ring size, by our current estimate, is usually around EU 52–55 (depending on the person, hand shape, and which finger the ring is worn on).

The most important rule for a surprise purchase: slightly bigger is safer

If you’re choosing between two options and you’re not sure, go slightly bigger.

Why?

  • In the important moment, the key is that the ring goes on the finger.

  • If the ring is slightly big, you can immediately secure it temporarily (clear adjuster, ring guard, other simple solutions).

  • Resizing later is common for most classic gold rings.

  • If the ring is too small, it can slightly spoil the moment.

6 proven ways to secretly find her ring size

1) “Borrow” one of her rings (the best method)

If she has a ring she wears on the same finger (or at least on the same hand), this is the best option.

How to do it:

  • Take the ring briefly (for example during a shower, workout, or while she’s sleeping).

  • Measure the inner diameter with a ruler or calipers (mm).

  • Or trace the ring’s inner circle on paper (see next point).

Note: for accuracy, measure the inner circle, not the outer edge.

2) Paper tracing: inner circle + measuring

If you can grab the ring but don’t really have tools:

  • Place the ring on paper

  • Trace the inner circle with a pencil

  • Measure the widest point of the circle (diameter in mm)

This can be surprisingly accurate if you trace carefully.

3) Take the ring to a jeweler

If you can keep the ring a bit longer:

  • Go to a jeweler or goldsmith

  • Ask them to quickly measure it with a ring sizer

This is very accurate and simple.

4) Use a “spy”: friend / sister / mother

The most elegant option:

  • Someone close to her casually asks her ring size

  • Or takes her to a shop “for fun” to try rings on

Bonus: you don’t have to borrow or measure anything yourself.

5) Measure while she’s sleeping (backup plan)

This only works if it’s realistic and safe:

  • Wrap a strip of paper or string around her finger

  • Mark the overlap point

  • Measure the length and compare it to a size chart

Important: string/paper can introduce error (too tight or too loose), so treat this as a backup method.

6) Compare it to your own finger (emergency tip)

Put the ring on your own finger and see where it stops:

  • Notice roughly where it fits

  • From there you can estimate a size range

Not the most accurate, but it gives you direction.

When to measure? Small details that make a big difference

Fingers change depending on time of day and temperature:

  • In the morning, fingers can be smaller; in the evening, larger.

  • In cold weather, fingers can shrink slightly; in warmth, they can swell a bit.

That’s why for a surprise purchase it’s completely okay to choose a size that leaves some room.

If you don’t know the size at all: the safest choice (based on Keefirivunts experience)

If you have to choose completely “blind,” very often this works:

  • 16.5–17.5 mm inner diameter

  • which is about EU 52–55

“But what if it’s wrong?” What to do after the proposal

If the ring is slightly big

  • A clear ring size adjuster helps immediately so the ring doesn’t spin or slip off.

  • A jeweler can also add small helper solutions that keep the ring more stable.

If it needs a proper resize

Resizing is common, and many rings can be resized (depending on design and setting). In general, it’s often possible to adjust a couple of sizes up or down, but it always depends on the specific ring.

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