Take a piece of non-stretchable string, ribbon, thread or thing strip of paper. Wrap it comfortably around your desired finger, close to the knuckle.

With a pen or marker, mark very thinly the spot where the ribbon overlaps, making sure you mark both parts of the ribbon

Unwrap the marked length of ribbon. Take a ruler and measure between both marks on the ribbon in millimetres or inches. Enter the measurements above to find out your ring size