Unlike shoe size or clothing size, frame size is not a hard and fast rule as simple as remembering one number. Frame measurements can be helpful in finding a good fit but there are some tricks to make them more useful. Let’s break down the numbers.
The first number of a frame measurement (51-18-140) is the width of one lens measured in millimeters. Look at this number to give you a sense of a frame’s overall width.
As a rule of thumb your eyes should be nearly centered in the width of the lenses. A frame that is too wide will make your eyes look close set while a frame that is too narrow will exaggerate the width of your face.
Bridge width is the second number in the string (51-18-140) and is also a factor in determining fit. The bridge measurement is the distance between the lenses.If you have a narrow bridge or close set eyes you will want this number to be lower (15mm to 18mm) if you have a wider bridge or wide set eyes this number can be higher (18mm to 22mm).
Also consider the location of the bridge in relation to the browline of the frame. Frames with bridges that are even with the browline (Jack, Dash, Sam) are great if the bridge of your nose is higher on your face (above your pupils). Frames with a lower bridge and a more arched browline (Reese, Tracy, Gretta, Hugo) are best if the bridge of your nose is lower on your face (approximately even with your pupils).
The third number in the string (51-18-140), temple length is the measurement in millimeters of the ‘arm’ of the frame. This measurement does not vary as much as the others with 135mm, 140mm, 145mm and 150mm being most common.
Acetate temples can be adjusted to get the best fit but it is important to have enough length so the temple sits horizontally and does not tip up over the ear.