How do you find out her ring size without her knowing?

Borrowing an existing ring is the most accurate way to size in secret. Choose one she actually wears on the ring finger of the relevant hand, since finger size varies from finger to finger and a ring from the right hand usually runs around half a size larger than the same finger on the left. Take it to a jeweller, who can read the size off it in a couple of minutes, then return it before it is missed. If you cannot get the ring out of the house, two quick methods capture the size at home: trace the inside circle of the band on a piece of paper with a fine pen, or press the ring flat into a bar of soap or some firm putty to leave a clean impression. Either gives a jeweller enough to work from. Pick a ring she wears less often so its absence does not get noticed.

If borrowing a ring is not an option, bring in someone who can help. A close friend, her mother or a sibling can often find the size naturally, by asking in passing, going jewellery shopping with her, or checking a ring she owns. You can also go ring shopping together under the cover of buying a gift for a relative and let her try styles on, which reveals both her size and her taste. Measuring her finger with string or a paper sizer while she sleeps is possible but unreliable, since any pressure changes the reading, so treat it as a last resort. As a rough fallback only, the average ring size for women in Australia sits around N to P, but build and finger shape vary so widely that an average should never be the basis for an actual order.

If there is genuinely no way to get the size discreetly, do not gamble on a guess. A ring can be resized later, but not always easily: plain bands move a size or so without trouble, while full eternity and pave bands, where stones run around the band, resize poorly or not at all. The cleaner answer is to propose with a placeholder or token ring, a simple stand-in that holds the moment, then choose and size the real ring together afterwards. The proposal still happens as a surprise, and the ring she wears for life is then guaranteed to fit and to suit her taste. Many jewellers, including bespoke makers, also include a first resize, which covers small errors once the real ring is on her finger.