Using a measuring tape (or piece of string, to be measured against a ruler later) is an accurate way to find your ring size in inches. Wrap the measuring tape around where you want the ring to fit on your finger. You want the tape/string to feel snug, but not tight. Use a pen to mark the tape or cut the small piece of string. Then, simply measure the string, or see how many millimeters it took to wrap around your finger. Finally, simply reference our ring size chart.
Note: The best time to try this is at the end of the day, when your fingers are likely to be at their largest.