Choosing the right ring size is important, so here are two techniques for measuring yours. Inside Diameter: Take an existing ring that fits your finger and measure the inside diameter. Finger Circumference: Use a tape measure to measure the circumference of your finger. If you don't have one, use a piece of paper or string, no thicker than 0.5 mm, and measure the length of the paper/string with a ruler. Use the table above to compare your measurements in mm.