The easiest way to find your ring size is to measure the inner diameter of a ring that already fits you well. Use a caliper for the most accurate measurement, or a ruler if you don’t have one.
Alternatively, you can measure the circumference of your finger by wrapping a narrow strip of paper around it and marking where the ends meet. Measure the length of the strip, then divide the circumference by 3.14 to calculate the diameter.
For the most accurate fit, you can also visit your local jeweller to have your ring size measured professionally.