Most eyeglasses have the size measurement printed on the inside of the left temple (the piece that goes behind your ear). This usually consists of three numbers – sometimes only the first two are printed – for example: 50 20 150.
The third number in the sequence represents the temple length; it is the measurement in millimetres of the 'arms' of the frame.
The first number in the sequence refers to the width of the lens (50-20-150).
The front measurement includes the diameter of the lens and the width of the bridge, measured in millimetres and usually separated by a small dash or rectangle (e.g.: 50 - 20). The lens diameter is the width of the lens, measured from the bridge, and the width of the bridge is the distance between the lenses above the nose, as shown in the figure below. Sometimes, the vertical measurement of the lens and the length of the arms are also included.
The second number in the sequence refers to the bridge width (50-20-150). Typical widths are 12-26 millimetres. The bridge measurement is the distance between the lenses.