Finding the best fit
The measurements of your current glasses can be obtained from the inside arm, and are typically shown as 53  18 135, these numbers being the lens diameter, bridge size and arm length.
If for some reason your glasses do not have this information, this can be obtained quite easily with the use of a ruler.
Lens Diameter: This is measured across the lens from the bridge
Bridge: This is the area which sits directly above your nose
Arm Length: There are three arm lengths that are generally regarded as standard; 135mm, 140mm and 145mm
There may be other numbers and codes on the inside of your glasses, please do not worry about these as they will be related to the manufacturer and are simply there to differentiate between different frame styles and colours.