A ring size is measured using a strip of paper or string and a ruler.
Place the strip of paper or string around your finger so that it overlaps in the center.
Measure to see which length corresponds to your finger size, and write this down on a piece of paper.
Alternatively, measure the circumference of your finger with a piece of string or thread, then divide by 3.14 to get your diameter in inches. Now just look up the conversion from inches to ring size in a table for rings online.
So this is how to measure ring size.