You have decided to propose, you are keeping it a secret, and you have hit the first practical hurdle: what size is their finger? It feels impossible to find out without giving the game away. It is not. Here are the reliable methods, from the classic to the cunning, and the safety nets that mean a small error need not spoil anything.

The gold standard: borrow a ring

The single most reliable method is to borrow a ring they already own and wear, then have it measured. A few important details make the difference between success and a near miss. First, the finger: an engagement ring is worn on the fourth finger of the left hand, so borrow a ring they wear on that finger, or at least know which finger the borrowed ring belongs to, because sizes differ noticeably from finger to finger. Second, timing: slip it away when it will not be missed, perhaps overnight, and return it before it is.

Once you have it, you can trace its inside diameter, take it to a jeweller, or measure it against our free precision ring sizer. We post the sizer to any UK address on request, so you can do this quietly at home.

The discreet delivery trick

Here is a small piece of cunning. Order the free ring sizer, or have any sizing tools delivered, to your workplace or a friend's address rather than home, so no unexpected parcel arrives to raise an eyebrow. You can then measure a borrowed ring, or take a discreet impression, entirely out of sight. A little planning here removes the main way surprises get spoiled: the tell-tale delivery.

Recruiting allies

You do not have to do this alone, and the right accomplice makes it easy. A close friend, sibling or parent can often find out a size through a natural conversation, or simply knows it already. A particularly reliable ally is a jeweller the person already visits, who may have their size on record. Bringing one trusted person into the secret is far safer than a risky solo guess, and it makes the whole thing more fun.

The methods to treat with caution

Some popular tricks are less reliable than they sound. Measuring a finger with string or a strip of paper while they sleep is a staple of films, but skin compresses, angles mislead and the knuckle complicates everything, so it is easy to be a full size out. Guessing from your own fingers rarely works. These can get you into the right region, but treat them as a rough guide rather than a final answer, and lean on the safety nets below.

The safety nets

Here is the reassurance that takes the fear out of the whole thing. You do not need to be exactly right on the day. If you are between two plausible sizes, choose the slightly larger one, within reason: a ring that is a touch loose can usually be worn carefully until it is adjusted, whereas one that will not pass the knuckle cannot be worn at all. And most engagement rings can be resized after the proposal, so a small error is easily put right. The main exception is a full eternity band, which cannot be resized, though these are rarely chosen as a surprise engagement ring. Propose first; perfect the fit afterwards.

Frequently asked questions

How can I find my partner's ring size secretly?

The most reliable way is to borrow a ring they already wear on the correct finger and have it measured, ideally with a proper ring sizer. A trusted friend or our free sizer, delivered discreetly, both help.

Which finger does an engagement ring go on?

Traditionally the fourth finger of the left hand. Sizes differ between fingers, so make sure any ring you borrow is worn on that finger, or note which finger it belongs to.

What if I get the ring size slightly wrong?

It is easily fixed. Most engagement rings can be resized after the proposal. Order slightly large rather than small if unsure, as a loose ring can be worn until adjusted. Full eternity bands are the exception, as they cannot be resized.

Are string or paper methods accurate?

Only roughly. Skin compresses and the knuckle complicates the measurement, so these can be a full size out. Use them as a guide, and rely on a borrowed ring or a proper sizer for the real answer.