Let's face it - we don't necessarily know our ring size, until we find one we want to buy. I know I never knew mine until I began making jewelry. That can make it challenging to shop online for rings, but there are solutions!
Many of my customers don't know what size ring they need and, since we're talking online, I can't measure their fingers personally. So, the first thing I recommend they do is go to a local jewelry shop and ask if they'll tell them their ring size. They'll likely take out a set of steel rings, in graduated sizes, and let you try them on until one fits just right. They'll look at the size of that ring and, voilà! You now know your size.
A few things to remember, though. Each finger is a different size, so you need to decide which finger you're asking about. Also, sizes can differ between your two hands. AND fingers tend to swell in hot weather. So, bear that in mind when deciding on your perfect size.
There's also a difference in the size you need for a thin band vs. a wide band. For wider bands, you need to go up about a half-size. So, if you fit a size 7 with a narrow band, you'd likely need a 7 1/2 in a wider band.
If there are no jewelry stores around or you just don't feel like taking that route, you can buy little plastic finger-measuring gizmos from many jewelry makers, myself included.