On your existing pair of eyeglasses you may have noticed certain numbers on the arm. These indicate the dimensions of your eyeglasses frame and are commonly visible on eyeglass frames.
The numbers of interest to us in the example above are 49 – 21 – 140
49 (mm) is the Lens Diameter
21 (mm) is the Bridge Width
140 (mm) is the Arm Length
Frames generally have five different sizing measurements. In addition to the three mentioned already there are another two.
Frame Width – which is the width of the glasses across the face and is used to indicate whether frames are Narrow, Medium or Wide
Narrow - Narrow (Small) Frame – Frame Width < 119mm
Medium - Medium Frame – Frame Width is between 120mm and 129mm
Wide - Wide (Large) Frame – Frame Width is over 130mm
The other measurement Frame Depth is used to determine whether a frame is deep enough to be able to mount a progressive lens.
Frame Depth > 28mm then the frame is suitable for progressive lenses
Frame Depth <= 28mm then the frame is only suitable for single vision lenses.