Most customers compare measurements from a frame they already have. Eyeglass frame measurements consist of three numbers in a format like 55-16-140. 

THE FIRST NUMBER (ex.54) = the width of the lenses. The frame of your glasses should be slightly wider than your face. 

THE SECOND NUMBER (ex.15) = the distance between the lenses or bridge width. The range is from 14 to 24. For instance, if your nose is narrow or your eyes are close together you'll want a lower number, such as 14mm to 18mm. 

THE THIRD NUMBER (ex.140) = the length of the temple arm including the portion going behind the ear. Temples should extend straight back to your ears, touching your face just in front of your ears. The arms need to be long enough to curve at the ear.