Nas Nakit offers a convenient Ring Sizer Tool to help you match your ring size.
How to use the Ring Sizer Tool
STEP 1) Download the free printable Ring Sizer tool here.
STEP 2) Print your Ring Sizer Tool on A4 paper at 100% sizing.
STEP 3) Once printed, slide an existing ring over the circles and match it to the correct size. If you don't have a ring, print the ruler in the PDF and hold it around your finger to determine the diameter and finger size. Remember to set your page scaling to "None" in print settings and check the ruler measures exactly 10cm when printed.