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Get one of her friends or sisters (that doesn't have mouth diarrhea oh), she'll help you out! or if your wife-to -be is the kind that wears ring, you can 'borrow 'it without her knowing.

Good luck!

Or measure her while she's sleeping

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dablazor:

Doesn't seem be an option as she isn't much of a jewelry type, just earrings; bracelets and necklaces on occasions.

What abt u take her to any shops like H&M or Foreve21 they sell fancy stuffs there..get her some

Or u can ask her close friend to do it for u..

Estimate and buy, let her wear it anywhere it fits.

Wrap a string round her finger and use it.