To measure ring size you can use one of two methods:
Method 1: using a ring you already own and that fits you well, measure the inside diameter at the widest point. Please use millimetres.
Method 2: Using a soft tape measure or sewing tape, measure around the finger of choice at the base of the finger. Please use millimetres. If you don't have have a soft tape, cut a strip of paper that is long enough to wrap around your entire finger at the base and mark the point where they meet. Please be sure to measure the correct end. You can then take the piece of paper to a ruler and measure the length in millimetres.