The critical question is not what size she is — it is whether the ring you are buying can be altered later. For many settings, it cannot.
A starting point. For a ring at this price, confirm with a jeweller before ordering.
Your size
Engagement rings are the highest-stakes sizing decision most people make. The ring is expensive, it usually needs to fit on a specific day, and — the part that catches people out — many of them cannot be resized as freely as a plain band.
That last point should shape the whole decision, so it goes first.
"It can be resized later" is true of some rings and badly untrue of others. Resizing works by cutting the band and either removing metal or adding a piece, then resoldering and refinishing. Anything that interferes with cutting the band limits or prevents it.
Ask the jeweller directly: how many sizes can this specific ring be moved, in each direction, and what does that cost? (what drives resizing cost.) The answer should shape how much effort you put into getting the size right, and whether a full eternity band is sensible for a surprise proposal at all.
If the proposal is a surprise, you are sizing a finger you cannot measure. The methods and their reliability are covered in full on the surprise sizing guide — borrowing a ring they already wear is the most reliable of them.
Two points specific to engagement rings:
Increasingly common, and it removes the problem entirely. Propose with a simple band or a temporary ring, then choose and size the real one together. If the ring is a full eternity design or otherwise unresizable, this is the sensible route.
A ring that will not go on cannot be put on her finger in the moment, which is the one thing you are trying to avoid. A ring that is slightly loose goes on fine. For a proposal specifically, the asymmetry between the two outcomes is much larger than usual.
An engagement ring and wedding band worn on the same finger behave like a single wide band. Two 2 mm rings together occupy 4 mm of finger and grip accordingly.
Because the two are usually bought months apart, this is easy to miss. When you buy the band, tell the jeweller it will be worn with the engagement ring, and if possible bring the engagement ring so it can be sized as a pair. Some jewellers will make a fitted or shaped band contoured to the specific engagement ring, which both fits better and sits flush. More on combined width
Two practical things that come up repeatedly:
For a ring at this price, home measurement is a starting point rather than the final word.
Ring sizing figures get copied between websites without anyone verifying them, so an error introduced once spreads quietly. Rather than ask you to take these numbers on faith, here is what stands behind them — each of which you can check.
Everything here is a close estimate rather than a guaranteed fit, and published charts differ between manufacturers by up to roughly a quarter of a size. Confirm the size against the specific retailer's chart, check whether the ring can be resized at all, and read the returns policy before ordering.
Usually not. Stones set all the way around the band leave nowhere to cut, so there is no material to remove or add. Some designs allow a very small adjustment; many cannot be altered at all and the only option is remaking the ring. Size a full eternity band correctly at purchase.
It depends entirely on the setting. A solitaire on a plain band typically moves a size or two either way without difficulty. A half-pavé band manages about a size. A full eternity or tension setting often cannot be resized at all. Ask about the specific ring before buying.
Guess large. A ring that will not go over her knuckle cannot be put on in the moment, which is the outcome worth avoiding most. A slightly loose ring goes on fine and can be sized down afterwards. Better still, borrow a ring she already wears on that finger. Ways to find out discreetly.
Not necessarily. Worn together the two behave like one wider band and grip more than either does alone, so the pair may need to be a little larger than the engagement ring was on its own. Tell the jeweller they will be worn together and bring the engagement ring if you can.
Typically one to three weeks, depending on the jeweller and the complexity of the work. If a ring is being sized close to a wedding date, allow for that in the timeline rather than assuming it can be done quickly.
Covert methods ranked by how often they actually work.
Sizing a set worn togetherTwo rings on one finger fit like a single wider band.
Between two sizesThe decision changes when resizing is not an option.
Women's size rangesWhere most orders fall, and why an average is a weak guess.