Step 1 - Get a non-stretchy string or paper about 6"(152MM) long and 1/4"(6.5MM) wide.  
Step 2 - Wrap around the base of your finger. It has to fit snug, but not too tight.
Step 3 - Mark the point on the string/paper where it overlaps forming a circle.
Step 4 - Measure the length of the string/paper from the starting point to the mark.
Step 5 - To determine your ring size in all countries open our Ring Size Conversion Chart for MM or Inches.

Ring finger measuring tips
If you are between sizes, order a larger size. Make sure your finger is at a normal body temperature -- fingers can shrink or expand when cold or hot. If your knuckle is much larger than your finger’s base, take two separate measurements and choose a size in between. You want a ring to fit over your knuckle, but not be too loose or it will shift around.