The first thing that you need to do is to get yourself a ring size chart. You can search for it on the Internet and print it out. For this, you will require a computer or laptop that has an active Internet connection. Launch the Web browser on your system and visit the Google search engine page. Search for a ring size chart and once you have found it, save it to your desktop. Connect the printer to your computer or laptop, install the necessary drivers, pull in the blank paper and take a print-out of the chart.
Now grab a piece of string and wrap it around your ring finger. It is the finger between your middle finger and the smallest finger of your hand. Make sure you wrap the strong around your ring finger loosely so that it does not stretch at all. The loop of the string should be loose enough to slip over your finger joint comfortably. You will know that the joint is just about right when it neither slipping too easily, not leaving any mark on your finger.
Mark the points on the string where the loop ends meet. This will be the size of the diameter of your ring finger. You may need someone to help you mark the string, as both your hands would probably be occupied.
Once you have marked the points on the string where the loop ends meet, lay it down on a flat surface and use a ruler to measure the segment between the two points that you marked in the previous step.
Consult the ring size chart to determine your ring size by looking up the length that you measured in the previous step on the size chart.