Check the inside of the temple arm for the size markings
Read the first number as lens width in millimeters
Read the second number as bridge width in millimeters
Read the third number as temple arm length in millimeters
Look for a fourth number if present, which may indicate lens height or frame size depending on the brand
Use the format: lens width – bridge width – temple length
Compare the measurements to your current glasses for fit
Ask an optician if the size markings are missing or unclear