1. You will need a string, ruler and a marker.

2 & 3. Wrap the string around your finger and mark where it meets. 

4. Place the string along a ruler and measure up until the mark where it met - This will give you the circumference measurement. 

OR

Simply measure the internal diameter of a ring you currently wear.

The best way to ensure the CORRECT SIZING, is to have your finger measured at a JEWELLER.