Wearers of prescription eyeglasses have probably noticed that a series of three numbers are stamped inside one side of their frame, which is also referred to as the arm of the eyewear. This is not the manufacturer’s attempt to leave you with winning lottery numbers, although that would certainly be nice. Nor is it a serial number or date of manufacture. So what are those numbers?

They’re the dimensions of your eyewear as measured in millimeters. Here’s how it breaks down.

Usually, the first number is the width of your lenses at their widest point, which can measure between 40 and 62mm in size. The next number is the bridge size, which measures the distance between the lenses and ranges from 14mm to 24mm. The last and largest number is the temple length, which measures from the frame hinge to the back tip sitting behind your ear. The temple length typically lands between 120mm and 150mm.

Eyeglass frame sizing numbers are measured in millimeters.Courtesy of Romi Eyewear

If you see other numbers, you may have the lens height stamped on the arm or possibly the model number of the frame. A square may also appear in between the numbers.

Opticians assess the right size depending on how the frame sits on your face. Ideally, your eyes should be centered in the lens opening, the nose balancing the frame, and the frame temples comfortable but secure enough to keep the eyewear on your head. A well-fitting frame is typically slightly wider than your face. Larger lenses can make your eyes look like they’re closer together, while narrow lenses can make them appear farther apart. The bridge width is determined by how comfortably the frames sit on your nose.

Does knowing your numbers guarantee a good fit every time? Not necessarily. Let’s say you have glasses that measure 42 mm by 20 mm by 140 mm. You have a second pair measuring exactly the same size, but one fits better on your face. The likely reason is a difference of frame style, which can provide a slightly different look or feel even if the numbers match up. Which is why trying on frames is always your best bet.