How to Find Your Partner's Ring Size Without Them Knowing: 7 Secret Methods

The most reliable way to find your partner's ring size without them knowing is to borrow a ring they already wear on that finger and measure its inside diameter against a UK size chart. If that is not possible, there are six more methods below, from recruiting an accomplice to simply sizing up and using free resizing as your safety net. Every Not Mined engagement ring is made to order in your chosen size and comes with free resizing after purchase, so even a near miss is fully recoverable. Here is exactly how to pull off the surprise.

In this article

• Method 1: Borrow a ring they already wear
• Method 2: The paper strip trick
• Method 3: Recruit an accomplice
• Method 4: The "try mine on" gambit
• Method 5: The casual shop visit
• Method 6: Use a ring sizer at home
• Method 7: When in doubt, size up
• UK ring size chart
• FAQ

Method 1: Borrow a ring they already wear

This is the gold standard. If your partner has a ring they wear on the ring finger of their left hand, borrow it briefly (while they shower, sleep or travel) and either measure the inside diameter with a ruler in millimetres or trace the inner circle carefully on paper, twice, to average out any wobble.

Two caveats. First, the ring must come from the correct finger: a ring worn on the middle finger or the right hand can be a full size or more different. Second, return it exactly where you found it. Nothing raises suspicion like a missing ring the week before a "surprise" holiday.

Match your measurement against the chart further down this page, or use our Ring Size Help guide for a full walkthrough.

Method 2: The paper strip trick

Cut a thin strip of paper about 10cm long. If you can manage it while your partner sleeps deeply (you know whether this is realistic), wrap it gently around the base of their ring finger, mark where it overlaps, then measure the length in millimetres. That is the finger's circumference, which converts directly to a UK size.

Honest advice: this method is famous, romantic and moderately risky. Fingers also swell at night, so if you do get a reading, treat it as half a size generous. It works best as a way to confirm a size you already suspect rather than as your only data point.

Method 3: Recruit an accomplice

A best friend, sister or mum can do openly what you cannot. A shopping trip that "just happens" to pass a jewellery counter, a casual "let's try rings on for fun" moment, or even a direct question framed as their own curiosity will usually produce the exact size, letter and all.

Choose someone who can keep a secret and brief them properly: you want the size of the left hand ring finger specifically. Bonus of this method: your accomplice can also quietly confirm which shapes your partner gravitates towards while they are at it.

Method 4: The "try mine on" gambit

If you wear rings yourself, hand one over. "Try this on, I wonder if it fits you" reads as idle curiosity, not reconnaissance. Note which finger it fits and how (snug, loose, only past the knuckle), then measure that ring of yours at home and adjust from the fit you observed.

No rings of your own? Compare hands directly during an ordinary moment. If their ring finger matches your little finger, measure a ring that fits your little finger. It is approximate, but combined with Method 7 it is usually close enough.

Method 5: The casual shop visit

Walking past a jeweller's window on a day out, wonder aloud what their ring size is. Most people do not know, and most will happily get measured on the spot "for fun". Done casually, months before you plan to propose, this gives you a professionally measured size with no suspicion at all. The trick is patience: do it too close to an anniversary or holiday and the game is up.

Method 6: Use a ring sizer at home

If subtlety has failed or your partner already suspects (or you are shopping together in all but name), just measure properly. Our Ring Size Help page covers how to measure accurately at home, including the best time of day (evening, when fingers are at their true size) and how to account for the knuckle.

Measure two or three times on different days if you can. Temperature changes finger size by up to half a size, so a single cold-morning measurement can mislead.

Method 7: When in doubt, size up

If your best intelligence still leaves you between two sizes, choose the larger one. A slightly loose ring goes on for the proposal photo; a too-tight ring does not go on at all. UK women's ring sizes most commonly fall around L to M, and men's around T to U, so if you truly have nothing to go on, those are the statistically safest starting points.

And here is your safety net: every Not Mined engagement ring comes with free resizing after purchase. Propose with confidence, then we will make it fit perfectly.

What size is that in the UK? Quick conversion chart

Inside diameter (mm)	UK size	US size
15.7	J 1/2	5
16.5	L 1/2	6
17.3	N 1/2	7
18.1	P 1/2	8
18.9	R 1/2	9

Measured a circumference instead? Divide it by 3.14 to get the diameter, then read across. Our Ring Size Help page has the full chart in every size.

Frequently asked questions

What is the average ring size in the UK?

For women, the most common UK engagement ring sizes are around L to M. For men, around T to U. These are averages, not guarantees, so use them only as a fallback when the methods above have failed.

Can an engagement ring be resized?

Almost always, yes. Plain and pave bands resize easily; full eternity bands (stones all the way round) generally cannot. Every Not Mined engagement ring includes free resizing after purchase, so a close guess is all you need.

Are UK and US ring sizes the same?

No. The UK uses letters (J, L, N and so on) while the US uses numbers. A UK L 1/2 is roughly a US 6. Always check which system a chart or jeweller is using before you order.

What if I propose with the wrong size?

Nothing is ruined. Slide it onto whichever finger it fits for the moment, enjoy the yes, and have it resized afterwards. With Not Mined the resize is free, so the only thing you risk is a funny story for the wedding speech. Talk it through privately →

Does it matter which hand I measure?

Yes. Hands can differ by up to half a size, and in the UK engagement rings are worn on the left hand. Always measure, borrow or trace from the left ring finger specifically.