How to read glasses measurements
Your glasses say 49. The pair you want says 51. Before you rule it out, discover what those numbers are actually telling you.
A 49 can be bigger than a 51.
You have found a beautiful frame. You compare the numbers on its temple with your old pair and pause. Different numbers: too big? Perhaps. But those three measurements alone are not the size of the whole frame. Let’s open the temple and read them.
The little code on the temple
Imagine it says 49 □ 21 – 145. These are millimetres: a lens width of 49, a nominal bridge width of 21 and temples 145 long. The square separates the values. No secret formula: three different parts of the same object.
Lens width
The horizontal width of one lens.
Bridge
The nominal distance between the two lenses.
Temples
The length of the temple to its tip.
The number that is missing
The 49 measures one lens. Around it, there is still acetate, the rim and the connections to the hinges. That is why a thick frame with a 49 mm lens can be wider than a slim metal frame with a 51 mm lens.
Even 49 + 49 + 21 gives you only 119 nominal millimetres of lenses and bridge. To know the outside width, you need the measurement of the complete front. The arithmetic works; it simply does not include everything.
Same bridge number, different fit
Two frames marked with a 21 bridge can rest differently on your nose. One has its contact area shaped from acetate; the other has two nose pads. Both the height at which they sit and the way they distribute weight can differ.
If your glasses slip, immediately looking for a smaller number can send you in the wrong direction. The shape of the contact area matters. Your optician can assess whether an adjustment or a different bridge would help.
145 millimetres, including the curve
Temple length follows the arm all the way to its tip. Measuring a straight line from hinge to ear gives a different distance: the curve is part of the story.
Use your old pair as a reference and pay attention to how it feels. Pressure at the temples, discomfort behind the ears or constant slipping are as useful as the printed number.
What if the writing has worn off?
Years of wear may have erased just those numbers. Look for the model name and photograph any remaining lettering. That can often help you find its specifications, checking the size variant too.
You can also use a ruler to measure the front from one outer edge to the other, without bending it. Record this as an approximate total width. It helps with comparisons, but does not automatically reveal lens, bridge and temple measurements.
Now you know what to ask
“I wear a 49” is a start. “This fits well, but I would like a shallower lens” is already much more useful. Put numbers and experience together, and the search becomes easier.
Our Pica, Posh and Corbs comparison makes these differences visible. For points of contact, continue to the fit guide. Your next pair can have different numbers from your old one and still deserve a try.