On the inside of your eyeglass frame, you will see numbers that tell you the frame measurement of your existing frame.

The temple arm length is usually written on the inside of one of the temples. The number is usually between 125 and 145, and represents the full temple length (including the bend) in millimeters (mm).

The eye size and nose bridge measurements are sometimes stamped in that same place, and are either separated with a dash ( - ) or a small box between the numbers. An example is 49-19, which would mean an eye size (widest part of the lens) of 49mm and a nose bridge (also called DBL or distance between lenses) measurement of 19mm. It is also very common for these to be stamped or engraved on the inside of the nose bridge.