Who knows what their ring size is!? It's like knowing your blood type off the top of your head. But there's an easy peezy way of finding out that doesn't require a trip to a jewelry shop. Or trying to guess from an existing ring placed over a chart.
You will need... a strip of paper, a piece of sticky tape and a ruler.
First - wrap the paper around your finger, stick it together, make sure there is enough wiggle room to get over the knuckle.
Second - cut the paper ring open and measure it flat. This gives you the circumference of the circle which you can use to look up the ring size.
Thirdly - consider how you wear your rings. I wear mine a bit loose, not so much that the ring is in danger of falling off, but there's movement. I do this because I tend to go up and down in size depending on temperature and my rings can get tight and uncomfortable. If they get too loose I switch them to another finger. Or I place a more fitting ring after it to wedge it on.
There are heaps of charts on-line that can help you work out the size you're after based on the circumference measurement or you can just flick me an email and I'll work it out for you.
Happy to help!