Measure your pupillary distance (PD)
This is the distance between your pupils and is required for us to make up your glasses perfectly, it is generally between 55 and 75mm. The average measurement for adults is 63mm.
Find a millimetre ruler & a mirror. Firstly, hold the ruler against your brow
Close your right eye and align the ruler's 0 to the centre of your left pupil
Close your left eye, open your right & read the mm line that lines up with the centre of your right pupil
Repeat the steps above twice to make sure the number is the same. This is your PD measurement