the string method
if you prefer a more hands-on approach, all you'll need is a small piece of string or a thin strip of paper & ruler or tape measure.
1. wrap the string or paper around the base of the finger where you'll want to wear the ring. make sure it's not too tight or too loose, allowing for a comfortable fit. mark the point where the string or paper overlaps, and then measure the length in millimeters.
2. once you have your measurement in millimeters, you can use an online ring size conversion chart to determine your ring size. different countries might use different sizing systems (US, UK, European, etc.), so double-check that you're using the right chart for your region.
i've found this conversion chart to be the simplest: