Take a strip of paper 2mm wide and 100mm long.

Wrap it around the base of your finger making sure it fits snug, but isn't too tight.

Mark the paper where it overlaps to form a circle.

Measure the length and compare it to the chart below (using this measurement as your circumference).

When taking your finger measurement, make sure you do it a few times over a period of a day in order to gauge an accurate size.