Ring Size Guide For guidance on ring sizes, please measure the inner diameter of one of the rings that you own. The number in milimetres will be your size. Please note that difference between sizes are small (up to ± 0.5mm will not be very noticeable). Therefore, often more than one size will fit well. In cases, where more than one ring will be worn together and the shape of the finger is not uniform, it might be useful to plan how the rings will be worn. In some cases, the size might differ depending on whether the ring will be worn closer to the proximal or middle phalanx of the finger. In case your are used to sizes in letters, please consult the table below.