The metric system is the standard measuring system used when measuring sunglasses. The measurements are always listed in millimeters (mm). When shopping at the Sunglass Museum you will see a measurement of each style listed in its description in a series of 3 numbers like this 54/18/145.
The first number is always for the eye size and is measured horizontally from the outside edge to the inside edge of one lens. The second number is always for the distance between the lenses and is the bridge/nose size. Lastly, the third number always represents the temple or arm length of the sunglasses.