Are Ring Sizes the Same on Both Hands?
You know her ring size—or you know the size of a ring she wears on her other hand. Before ordering, check whether those are really the same piece of information.
Which details should you write down?
Instead of saving “ring size 7,” save the hand, finger, chart system and, if available, inner measurement. A bare number leaves too much to guess later.
Cartier explicitly notes that the same fingers on different hands may differ. There is no fixed amount you can add or subtract to predict the other hand.
Is the dominant hand always larger?
Do not use that as a buying rule. Even if it is true for a person, it does not give you the exact measurement. Measure rather than assuming “right hand means one size up.”
If you are buying a gift and only have one reference ring, find out where she actually wears it. A ring in her jewellery box may be one she keeps but no longer wears because the fit is wrong.
How do you use an existing ring well?
Choose a round ring that fits comfortably on the intended finger. Measure the inner opening, following the seller's chart or a calibrated tool. Do not include the band thickness in the diameter.
If the ring is bent or open-ended, an inside-diameter reading may not represent the fit well. Use a fitting tool or ask a jeweller instead.
What if she wants to swap hands?
Check both fits in normal conditions. The ring should pass the joint without force and remain secure. If it slips off one hand, do not wear it there just because it is comfortable on the other.
For adjustable designs, follow the stated range and avoid frequent bending to make one piece serve several very different fingers. A second suitable ring can be better than repeatedly changing the same one.
How can you keep a surprise without guessing?
Ask which finger she likes to wear rings on during a normal conversation, or ask a trusted friend for a specific clue. Do not take or photograph private belongings without permission.
Read how to ask a friend for gift advice and how to measure an existing ring. A small amount of clear information can save a large amount of confident guessing.