How to Calculate Ring Size? Rings are the most intimate of jewelry, often worn daily, and complete our style. They should fit well on your finger and make you feel comfortable. Below, we offer some tips to help you determine the correct size for your ring; Take your finger measurement in the evening when your fingers are at their widest. Do not measure your fingers when they are cold; let them warm up to room temperature. The width of the ring is an important factor in determining the correct finger size. You can use one of the two easy methods below to find your ring size; Diameter Calculation Take a ring you have worn before. Use a ruler with millimeter (mm) measurements. Place the ring horizontally on the ruler and measure the inner diameter of the ring from end to end, making sure to measure the widest part of the ring. The value corresponding to the ring's mm on the ruler indicates the inner diameter of the ring. Finger Measurement Wrap a piece of string, wire, or paper strip around your finger. Mark the point where the two ends of the string meet with a pen. Obtain a ruler with millimeter (mm) measurements. Measure the string or paper up to the marked point on the ruler. The value you find will give you the circumference of your finger.