A - IF YOU KNOW YOUR SIZE:

If you know your size or finger measurement (in mm), you can go to any online ring conversion website and enter your info.
Here is an example of a ring size conversion chart: http://www.onlineconversion.com/ring_size.htm

B - MEASURING YOUR FINGER WITH A PRINTED SIZE CHART:

1. Right-Click on the image below and select Save Image As.
2. Print the image without scaling and ensure the length from the 'Read Size Here' line to the end is 3.5 inches.
3. Cut out the printed image, then wrap the sizer around the desired finger. Read the number next the 'Read Size Here' line. 

C- WITH A STRING / FLOSS:

1. Wrap the string/floss snug around your finger, make sure it's nice and tight and cut it carefully;
2. Measure the distance with a ruler (in mm); that's your finger's circumference;