Use a flexible measuring tape or length of string. Wrap the string around your finger so that the ring will be comfortable wear. If you’re creating a ring stack, consider where on your finger the ring will sit. Mark the point on the string where the ends meet with a pen. Lay the string on a flat surface and use a ruler to measure the length (millimeters) up to the mark. If you are between sizes, opt for the larger size.

Choose a ring that already fits your finger well. Measure the interior diameter of the ring in millimeters (mm). The interior of the ring should aligh with outer edge of the circle.