Frame measurements are important to keep in mind if you are wanting a precise fit. Thanks to a standard system of measurements, it is fairly easy to pinpoint a size that works for you.
Lens or eye width, bridge size and temple length are three main measurements to understand. They are all measured in millimetres.
This measurement determines the horizontal length of one lens. The range is 40-62mm, while the average size is 49-51mm. For sunglasses, bump your average up by at least 2 millimetres to accommodate for oversized lenses.
The bridge measurement is the distance between the two lenses. The range is 14-24mm, with the average being 18-21mm. If you have a small bridge, stick with a frame below 20mm. Keep in mind that a frame with nose pads is adjustable and will guarantee a better fit.
The temple length is simply the length of the two arms that extend to the ears and support the frame. Specific temple lengths should only be considered if you know that your ears are placed closer or farther from your nose, beyond what is noted as average. The range is 120-150mm, and the average adult size is 145-148mm.