Look for the size numbers printed on the inside of the temple arm
Read them in this order: lens width, bridge width, temple length
Lens width is the width of one lens in millimeters
Bridge width is the distance between the two lenses in millimeters
Temple length is the length of the arm from hinge to tip in millimeters
Example format: 52-18-140
If only two numbers appear, the frame may list lens width and bridge width only
Check the inside of the bridge or temple arm if the numbers are not on the first visible area
Use the numbers to compare with your current glasses for fit
Match the size to your face and comfort preferences