Ring finger measuring tips:

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 fingers 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.  

Measure your finger with a ruler :

Get a non-stretchy string or paper about 10cm long. Wrap around the base of your finger. It has to fit snug, but not too tight. Mark the point on the string/paper where it overlaps forming a circle. Messure the string or paper with a ruler and check the chart below.