A gold or platinum ring is expensive and can usually be sized up or down only two sizes, which is a matter of millimeters. Some rings can’t be sized as much or at all. Do you really want to take a chance that they won’t like it or it can’t be sized to fit them properly (if they don’t already know their ring size)?
Here’s how to properly size a finger: go to an independent jewelry store and get your partner’s finger sized with metal sizing rings, preferably a set that comes in quarter sizes. String, floss, paper and plastic don’t behave like metal, so it’s not surprising that using them to measure finger size ranges from somewhat off to highly inaccurate.
And using one of their rings to find out their size? Only if it is worn on the same finger as the ring you want to give. It should be obvious that finger sizes vary on a hand. Finger sizes also vary between the dominant and nondominant hand. And the width of a band and whether it’s comfort fit will affect the size needed as well. The correct way to size an existing ring is not to use a piece of string or a ruler or any method that involves tracing and paper, but to use a mandrel and/or digital calipers.
I’m guessing that because you don’t know their ring size, you haven’t discussed what they want in a ring. There are so many permutations of metal, stone size and shape, setting and band style that it’s unlikely that you would buy a ring that is exactly what they most desire. It is a burden to them if you buy them a ring and expect them to wear it for the rest of their life even if they don’t love it. We do not want to see your partner post about how excited they are to be engaged but how much their ring is not what they wanted.
You may also like this, Best Guide for Engagement Rings
How do you know the ring size before buying: