Nobody tells you that your ring size is not a fixed fact about your body. It moves. Heat, salt, the end of a long day, the decade you're in. The size that fit at twenty-eight is not automatically the size that fits now, and there is nothing wrong with either number.
So before you order: here is how to choose a size you'll still be wearing in five years, rather than one you take off after an hour.
How should a ring actually feel?
A correctly sized ring slides on with light resistance and needs a small, deliberate twist to come off over the knuckle. It should not spin freely when your hand is at rest, and it should not leave an indent in the skin after twenty minutes.
The knuckle is the part people get wrong. Size to the knuckle, not to the base of the finger - the ring has to travel over the widest point. If the knuckle is significantly larger than the base, take the knuckle measurement and accept that the ring will have a little movement once it's on. That is a correct fit, not a loose one.
If you already know your number, the full EU, US and UK conversion chart is here.
Why does my ring size keep changing?
Ring size varies by two to three EU sizes over a single day and across a year, for reasons that are entirely ordinary:
Temperature. Fingers swell in heat and shrink in cold. A ring bought in February can be uncomfortable in July.
Time of day. Fingers are smallest in the morning and largest in the evening. Measure at the end of the day.
Salt, alcohol and heat. All three cause fluid retention. A measurement taken the morning after a long dinner is not your size.
Exercise. Hands swell during and after activity. Measure at rest.
Age and hormones. Ring size changes across a lifetime, and perimenopause and menopause frequently change it again. If your rings stopped fitting somewhere in your forties and you quietly stopped wearing them, that is a sizing problem, not a message. Get remeasured and put them back on.
That last one deserves saying plainly, because a lot of women interpret a ring that no longer fits as evidence they've aged out of it. They haven't. They've changed size. It is the same category of problem as needing a different shoe.
When should I size up?
Go up when you are between two sizes, and go up specifically in these cases:
Wide bands. Anything wider than about 5mm - signet rings, rope bands, chunky stacking bands - sits tighter than a thin band of the same internal measurement, because it contacts more of the finger. Size up half to a full EU size.
Rings you'll wear daily. A ring that is perfect at 9am and tight at 9pm gets taken off, and a ring that gets taken off gets lost.
Warm climates and summer buying. If you're ordering in winter for year-round wear, size for summer.
Go down, or stay put, for a very thin band worn occasionally, and for a ring going on a finger with a pronounced knuckle where the base is much narrower.
Which finger?
Ring sizes differ between hands and between fingers by one to two EU sizes, and the dominant hand is usually the larger one. Measure the exact finger on the exact hand. "It fits my other ring finger" is how a ring ends up in a drawer.
As for which finger you should wear it on - there are no rules worth obeying here. Signet rings traditionally went on the pinky or the index. Stacking rings go wherever the stack looks right. A statement stone on the index finger is a genuinely underused move, because it's the finger people see when you gesture, and gesturing is most of what hands do in conversation.
Can gold-plated rings be resized?
Plain 925 sterling silver bands can usually be resized by a jeweller by one to two sizes. Gold-plated sterling silver should not be resized. Cutting and resoldering the band burns off the 18ct gold plating at the join, and the repaired section will not match the rest of the ring. Order plated rings in the size you need.
This is the practical reason to spend the extra minute measuring properly. A plated ring is not a garment you can take in.
Buying a ring for someone else
Borrow a ring she already wears, measure the inside diameter in millimetres, and note which finger it came from. Do not estimate from the look of her hands - nobody can do this, including jewellers.
If you can't borrow one, buy the piece in her most likely size and buy it from somewhere that will exchange it. And if you're buying for a woman who has been sizing herself down for years because she thinks a bigger number means something: buy the size that fits her hand.
The short version
Measure at the end of the day, at room temperature, on the exact finger. Size to the knuckle. Go up when you're between sizes, and go up again for a wide band. Don't plan on resizing anything plated.
Then wear it. The whole point of a ring you can afford to own several of is that it lives on your hand, not in a box waiting for an occasion that keeps not arriving.