I had no idea this was going to be such a confusing task. I've looked all over the internet for information and found that many of the charts out there are all over the map! To investigate this further, I also visited a professional jeweller at our local fine jewelry shop to compare the sizes on my rings and see if it matched the ring mandrel I just purchased.
Here is a chart I came up with that represents the most consistent numbers:
Here are the most common methods to determine ring size:
CIRCUMFERENCE METHOD (If you don't have a ring to reference)
1. Cut a narrow strip of paper 1/4" wide by 5" long.
2. Wrap it snugly around your finger. Use a pencil to draw a line where the ends overlap.
3. Find the closest measurement (in MM) on the sizing chart. This is the circumference of your finger.
EXISTING RING DIAMETER METHOD (More accurate)
1. Find a ring that fits your finger comfortably.
2. Take an accurate metal ruler and measure the inside diameter in MM.
3. Compare to the chart above.
SOME POINTS TO KEEP IN MIND:
1. If your knuckle is a lot larger than the base of the finger, measure both sizes and select a size in between.
2. If you're buying a wide band, you will probably need a size larger for a comfortable fit.
3. Your fingers are usually larger on your dominant hand, up to a full size.
4. The average woman's finger is size 6. The average man's finger is size 10.
5. Your fingers change size during the day, depending on your diet and may even be larger in summer when temps are higher or if you live in a humid climate.
6. If you can, get help with measuring for better accuracy.
7. Visit your local jeweller and they will be happy to help you try on ring sizers to get the most accurate reading.
Hopefully this will help you feel more comfortable buying rings online. Make sure to check the chart on each website you may be purchasing from because the numbers do vary. It certainly was a good exercise for me to make sense of all the charts out there and to try to come up with one for my shop.
I have to give credit to these sites for my research: Amazon.com, Bluenile.com and Zales.com.
Each of these sites have a page dedicated to determining your ring size.