Engagement ring sizing

1. 1

   Check for a ring she already wears on that finger

   A ring worn regularly on the left ring finger (or the finger you intend, in cultures where the engagement finger differs) is the best evidence available. Measure it with the screen sizer or a ruler, and note the band width — a thin borrowed ring does not predict a wide engagement band without the width adjustment.

2. 2

   If there is no such ring, ask — carefully

   A friend or family member may know the size, or may be able to ask in a context that does not raise suspicion (shopping together, trying on a friend's ring for fun). Treat a second-hand size as an estimate, not a measurement, and prefer a resizable design.

3. 3

   Choose resizable, and confirm the terms in writing

   Before buying, confirm: can this exact design be resized, by how much, at what cost, and how long does it take? Some settings (tension, full eternity, intricate gallery work) cannot be resized safely. If the answer to any of those is unclear, choose a different design for the proposal.

4. 4

   Size up, not down, when estimating

   An estimated size that is a fraction loose can be worn for the proposal and resized later. An estimated size that will not go on cannot be worn at all, and the proposal moment is not the time to discover that.

5. 5

   Plan the post-proposal fitting

   Agree — with yourself, before the proposal — that a joint fitting after the proposal is part of the plan, not a failure of it. Many couples treat the proposal ring as the design and the fitting as the final sizing step, which removes the pressure to guess perfectly in secret.

• Using a ring from another finger. A middle-finger or right-hand ring is not evidence for the left ring finger. If it is all you have, treat it as a range, not a size, and buy resizable.
• Buying an un-resizable design on a guess. Full-eternity and tension settings are beautiful and unforgiving. For a surprise, they require a real measurement, not an estimate.
• Forgetting the wedding band. An engagement ring will usually be worn with a wedding band. If the band will be wide, the engagement ring's fit with the band on — not alone — is what matters long-term, and a fitting after the proposal should test both together.
• Measuring once, in secret, under pressure. A rushed, single, secret measurement is the least reliable kind. A borrowed ring measured carefully at home, twice, beats a hurried finger measurement every time.

This guide covers sizing, not buying: stone, metal, setting and budget choices are outside its scope. It also cannot solve a surprise with no measurement and an un-resizable design — in that combination, the honest answer is to change one of the two (a resizable design, or a placeholder proposal followed by a joint fitting).