Use a flexible measuring tape and wrap it around your finger close to the knuckle. This is the thickest part of your finger, and you want the ring to slide on with ease. If you don't have a measuring tape, a piece of paper can be used. Don't wrap the measuring tape or the piece of paper too tight, you want to ring to feel comfortable.
Record the measurement where the tape or piece of paper overlaps. If you are using a piece of paper, measure it using a ruler.
A sizing chart is available below in millimeters. Convert your measurement and compare against the sizing chart. If you fall between 2 sizes, choose the larger size.