Cut a strip of paper 15cm in length and 0.5cm wide.
Alternatively, use a string.

Wrap it around the knuckle of your finger and use a pen to mark the point on the paper (or strinf) when it forms a circle.

Lay the paper (or string) flat and measure the distance from the end of the paper to the mark you made.
This is the distance to the circumference of your ring.

Match your measurement with the Ring Size Conversion Chart to determine your ring size.
Take the larger size you if you are in between sizes.