What is the average ring size to guess if I have no way to measure?

For US sizing, roughly 6 to 7 is typical for women and 9 to 10 for men, though the spread around those figures is wide enough that a guess is a last resort, not a plan. Order from a retailer with free resizing if you have to guess.

Which finger should I measure?

The specific finger on the specific hand the ring will be worn on. Ring finger size can differ noticeably from other fingers on the same hand, and a person's dominant hand is often very slightly larger than the other.

Is it true a ring should be measured while someone sleeps?

Slipping a ring off a sleeping partner's finger to trace or size it is one of the more common tips given for this, and it works, but it carries an obvious risk of waking them or of the ring not fitting back on unnoticed. Borrowing a ring during the day, from a jewellery box, is lower risk for the same result.

What if they never take their ring off?

Ask someone close to them, a sibling, a parent or a close friend, who may already know or be able to find out more naturally than you can. Failing that, a size that runs slightly large is the safer error, since a ring that is a little loose can be resized down more easily than one that will not go on.

How to Find Out Someone's Ring Size Without Asking

Asking gives the game away, and guessing is expensive to get wrong. There is a middle path, and it does not need them to be involved at all.

The reliable method: borrow a ring they already wear

The most accurate way to find someone's ring size without telling them is not to measure their finger at all. It is to find a ring they already wear regularly on the correct finger, and measure that instead. A worn ring already accounts for the things that make a finger tricky to size directly, most of all the knuckle it has to pass over, so copying it sidesteps the hardest part of the problem entirely.

Look for a ring worn daily on the same hand and finger the new ring is meant for. A dress ring worn only occasionally is a weaker signal, since it may already be a slightly loose or tight fit that its owner tolerates for an evening but would not choose for daily wear.

Measuring the borrowed ring

Once you have it, measure the inside diameter, not the outside: the inside is what determines fit, and a wide band can add a millimetre or more to the outer measurement without changing the inside at all. Lay the ring flat on the ring sizer on this page and match its inner edge against the circles until one lines up. Take a photograph of the match before returning the ring, in case you need to double check later.

Return the ring exactly where it came from and exactly how it was left. This step matters more than it sounds: a ring left in the wrong spot, or a jewellery box left open, is the most common way this plan gets discovered.

If there is no ring to borrow

Without a ring to copy, the options get less precise, in roughly this order of reliability:

Ask someone who would know. A close friend, sibling or parent may already know the size, or be able to ask in a context that raises no suspicion at all.
A discreet direct measurement. Wrap a strip of paper snugly, not tightly, around the base of the finger while they are asleep or otherwise occupied, mark where it overlaps, and measure the strip afterward against a ruler. It is less accurate than a borrowed ring, since a sleeping hand and a waking one are not identical sizes, but it beats an outright guess.
A sized-up estimate. As a last resort, order slightly larger than your best guess. A ring that needs to come in a half size is a routine, inexpensive resize for most jewellers; a ring that will not go on at all is a return.

What to avoid

Skip online "average size by height or hand span" charts. They exist and get shared often, but the correlation between hand size and ring size is loose enough that they are barely better than a flat guess, and a wrong confident number is worse than an honest uncertain one, because it removes the instinct to hedge toward the safer, slightly larger size.

Avoid measuring a finger with string rather than paper. String stretches under tension, which reads a size or more small, and it is the single most common way a DIY finger measurement goes wrong, independent of who is doing the measuring.

Buying with room to correct

Whichever method you use, the number you end up with is an estimate, not a certainty, and the purchase should be made accordingly. Most jewellers offer a free or low-cost resize within the first 30 to 60 days, and it is worth confirming that policy before you buy rather than after. A ring bought with a resize option removes almost all the downside of being a size out, which a borrowed-ring measurement rarely is by more than that anyway.