If you are buying ring online, sometimes you are not sure how to find out your size. To those who are already get used to ring size chart, it's better. But its a new thing for you and you don't feel like cutting a piece of paper, and wrap it around your finger and count the length and match with the size, I have a quick tip to measure the diameter of your finger.
Basically, grab a ruler and place it on the finger you want to measure, like a photo above, and voila! If you see double, try to peek with one of your eyes.