The answer is simple. If you use your old glasses as a template to obtain the overall width and lens height .
They do not even have to be prescription glasses. If you have a pair of sunglasses that fit you well then these will do fine.
One of the most important measurement is the overall width of the prescription glasses.
For some unknown reason this measurement is not included on the arm of the glasses as the lens width bridge and arm length is.
We would suggest that you measure the Total Overall width of the glasses frame from outside to outside and use this measurement when searching for glasses.
A big point to note is that generally a few millimeters either way is not a problems so you do not have to be precise. Prescription Glasses are designed to be bent and adjusted to suit your face.
At AusSpecs we catagorise our eyeglasses into:
If you are uncertain of how to measure your glasses please check this link How to Measure My Glasses