You can easily and confidentially measure your own finger size from 0-17 with accurate full, half, and quarter sizes. You can choose between the free PDF download (please provide your email address) or purchase our Multisizer sizing finger belt. We recommend the latter as that would be the most accurate, however, as long as the size is within a +/-1 size difference, that would be fine (only applicable to non-eternity designs).
If you don't want to ruin the surprise, you can always use either source to measure a current ring that is being worn. Make sure to take note of which finger and hand it is being worn on as each hand fluctuates in sizes.