Many brands such as Persols, Pradas, and Ray Ban Sunglassses commonly use the same model number but make several different sizes. For example, the Ray Ban Aviator Large Metal sunglasses all have the model number of RB3025LM. However, they come in 3 sizes - 55mm, 58mm and 62mm wide.
So how do you determine which is the the size and model of your current frames? You can measure them at their widest point as the crow flies. However, with two to three millimeters difference it can be hard.
Most sunglasses have writing on the arms or under the nose bridge that should provide the required information. Additionally, the information may be on your original packaging. However, it's always best to check your frames as we have seen many major department stores pack the sunglasses in the wrong boxes.
The Industry Standard and The Sunglass Fix measure lenses at the maximum with as the crow flies. Not along the curve of the lenses.
1) The maximum width in Millimeters (convert inches by making into decimal and multiplying by 25.4)
Example: 2 and 3/8's of an inch becomes 2.38 inches * 25.4 = 60mm
2) As the crow flies, not along the curve of the lens
Manual Measurements using calipers or a ruler: Allow 1/2 to 2 mm for lens portion under rim.