How to measure frame size: the numbers that decide the fit

The three numbers on the temple

Almost every frame carries three numbers stamped inside the temple arm or on the bridge, usually written as 52-18-145. The first is lens width, the horizontal size of a single lens in millimetres. The second is the bridge, the gap between the two lenses. The third is temple length, measured from the hinge to the tip of the arm. Those values describe a frame far more precisely than a label such as small, medium or large, and they are the only reliable way to compare two models from different brands.

Total front width matters most

Lens width alone can mislead, because two frames with identical lenses can differ by ten millimetres overall. Add lens width twice to the bridge and then to the thickness of the rims to estimate the total front width, and compare that number with the distance between your temples. A front that is wider than your face slides down and rests on your cheeks; a front that is narrower pinches and leaves marks in front of the ears after an hour of wear.

Use a frame you already own

The fastest method is to measure a pair that already fits, ideally with a ruler laid flat across the front. Note the three numbers, then look for listings within two millimetres on lens width and one millimetre on bridge. Pupillary distance, which your optician records on any prescription, is the other reference point: the optical centre of each lens should fall in front of the pupil, and a bridge that is too wide pushes those centres outwards.

Vintage sizing is not modern sizing

Frames from the seventies and eighties often run narrow in the front and long in the temple, while many current models do the opposite. On LUNETOS every listing states lens width, bridge, temple length and total front width taken from the actual item rather than from a catalogue sheet, so a vintage reference can be compared with a current one on the same basis. When a measurement is missing, ask the seller before buying: it is a normal question and a serious seller answers it in minutes.

Small adjustments a professional can make

Temple arms in acetate can be gently reshaped with heat, metal arms can be bent behind the ear, and nose pads on metal frames can be tightened or replaced to lift a front that sits too low. What cannot be changed is the width of the front or the bridge, so those are the two measurements to get right before you buy. Anything else is a twenty-minute adjustment at an optician and costs very little.