Using a measuring tape is the best and most accurate method to determine your ring size. For the best results, make sure to give enough space to accommodate your knuckle. Keep in mind that your fingers vary in size during the day depending on several factors, like temperature and humidity. Therefore, it is recommended to measure your finger at the end of the day, when it is likely the largest. Also, the same finger might differ in ring size for your left and right hand, make sure to measure the finger that you are planning to wear the ring on.
Step 1: Place the measuring tape
Place the measuring tape over your finger and pull it around into a circle.
Step 2: Adjust the size
Adjust the size of the circle, move it across your knuckles, and make sure it fits comfortably.
Step 3: Note the measurements and find a size
Take note of the measurements in millimeters and use the comparison chart or measurement guide PDF (attached) to find your ring size.
Apart from using a measuring tape, there are several other ways to find your size. The video below show a few different ways.