Whether you want to buy the special someone in your life a ring for a proposal, a birthday gift, a graduation present or as a "just because" gift, you must find out your friend or romantic interest's ring size before you can make the purchase. Of course, you can't ask the recipient directly because that would spoil the surprise. Instead, employ one or several sneaky tactics to find out the ring recipient's size without them knowing.
Remove a ring from her jewellery chest if you live together or you spend a lot of authorised alone time at her home. Choose a ring you have seen her wear on the third finger, also known as the ring finger, of her left hand if you're purchasing an engagement ring. Hold it against the different circles on a ring sizing chart until you find a match. Replace the ring before she gets home.
Wrap a length of twine around his finger while he is in a deep sleep. Use a light, gentle touch, and don't attempt this method if he's a very light sleeper. You could also try convincing him to use the string-around-the-finger method to remember obligations. After he discards one of the strings he used, recover it and hold it against a sizing chart.
Contact the ring recipient's parents or siblings. At least one of her immediate family members has purchased a ring for her as a gift at some point and likely still knows her ring size. Remember to only ask a trustworthy, nongossipy person, and ask them to please not ruin the surprise.
Ask the recipient to play a Newlywed Game style personal trivia game. Write a different random personal question on each index card. Make one question about ring size. As each card is played, both players write down their guesses about the other player. Reward a point for each correct answer. At the end of the evening, you'll have had a fun game night and you'll know more about the recipient, including his ring size.
If none of these methods work, go with a half-size more than the average ring size for the recipient's sex. A too-big ring is easier to resize than one that is too small. Buy the ring from a jeweller who includes ring sizing for free with the purchase. If you don't have a ring sizing chart, you can download a printable one from the Internet or trace the inside of the ring you borrowed and then bring it to the jeweller.
Be very careful if you decide to ask friends about the ring recipients' ring size. Make sure you only ask trustworthy friends who may actually know. The more friends you ask, the more likely it is that news of your gift will reach the recipient prematurely.