Your finger is a different size than it was this morning
Fingers swell and shrink through the day, with heat, with salt, with exercise, with altitude on a long flight, and with the menstrual cycle. The variation is small in absolute terms and large relative to the tolerance of a ring.
Which means the question is not what size your finger is. It is what size it is most of the time, and a measurement taken at the wrong moment will be confidently wrong.
Learning how to measure ring size at home is mostly about controlling for that, and it takes one afternoon rather than one minute.
The rules that matter more than the method
Measure at the end of the day. Late afternoon or evening, when your hands have been in use and are at their usual size. First thing in the morning is the smallest they will be.
Measure at a normal temperature. Not after a hot shower and not after a walk in the cold. Both directions produce a reading you will regret.
Measure the correct hand. Fingers on the dominant hand are usually slightly larger. The same finger on each hand can differ by most of a size.
Measure more than once, on different days. Three readings across a week tell you the truth. One reading tells you about one afternoon.
The method that works
If a ring is already worn on the correct finger, that ring is the best instrument you have. Measure its internal diameter across the middle in millimetres, or take it to be sized properly against a mandrel. This is more accurate than measuring the finger itself, because a ring that already fits has resolved the whole problem for you.
If there is no ring, the reliable home method is a strip of paper, not string. Cut a narrow strip, wrap it round the base of the finger snugly but not tightly, mark where it overlaps, lay it flat and measure the length in millimetres. That length is the circumference, which converts to a size.
Then do the knuckle check, which people skip. Pull the paper strip up over the knuckle. If it will not pass, the ring has to be sized for the knuckle rather than the base — and a ring sized for a larger knuckle will rotate on a narrower base unless something is done about it at the making stage.
What misleads people
String. It stretches and it compresses. Every string measurement reads small.
Printable paper sizers. They work only if the page printed at exactly one hundred per cent, and printers are frequently doing something else. If you use one, check it against a ruler first — there is usually a scale bar on the sheet for this reason.
Measuring over a knuckle without noticing. A common error that produces a ring which spins.
Assuming band width does not matter. A wide band fits more tightly than a narrow one at the same size, because it covers more of the finger. Anything approaching six millimetres of width usually needs a slightly larger size than a thin band. Say what width you are ordering when you give the size.
Sizing someone else, without asking
The problem specific to a proposal, and it has one reliable answer: borrow a ring she already wears on that finger, for a day.
If borrowing is impossible, press one into a bar of soap or a piece of putty and measure the impression. Photographing a ring next to a ruler is less reliable than it looks, because the angle distorts it, but it is better than guessing.
What does not work is comparing to your own finger by feel, or asking her mother, who will be confident and frequently wrong.
And if you cannot establish it at all, size slightly large rather than slightly small. A loose ring can be worn on the day and adjusted afterwards. A ring that does not go on is a much worse moment. The entry on surprise commissions covers the rest of what has to stay hidden.
What cannot be resized later
Worth knowing before you choose a design rather than afterwards.
A plain band is straightforward to resize within a range. A band with stones set all the way round — a full eternity ring — usually cannot be resized at all, because there is no plain metal to cut and rejoin. Rings with pavé running well down the shoulders are limited for the same reason, and a contoured wedding band shifts out of alignment if its circumference changes.
Deep engraving also restricts where a join can be made without disturbing it.
None of these are reasons to avoid those designs. They are reasons to be more careful about the size the first time, and to say out loud at the design stage if your hands change noticeably through the year.
If it is being made rather than bought
A commission has an advantage here. The size is not selected from stock, so it can be confirmed late, and the band width and profile are known when the size is set rather than discovered afterwards.
If you would rather not measure at all, a sizer can be sent with the collection kit on request, which removes the guesswork entirely.
And tell us if you work with your hands, if your fingers swell in the heat, or if you have a knuckle that a ring has to travel over. Each of those changes what should be made. Say it at the start rather than at the fitting.