Measure your ring circumference 1. Wrap a strip of paper around your finger. 2. Make sure that the paper is pulled snug to your finger, the tighter the better. 3. Mark the spot where the paper meets and measure the distance with a ruler (mm). 4. Use the chart below to determine your ring size. Measure your diameter 1. Select an existing ring that fits the desired finger. 2. Measure the internal diameter of the ring (in mm). 3. Use the below chart to determine your ring size.