Getting the Prescription Right Is Only Half the Job
Whether a pair of glasses is comfortable to wear depends only half on the prescription. The other half is whether the frame size actually fits your face.
Too big, and the glasses keep sliding down, the lenses get thicker, and the optical center drifts out of place. Too small, and the frame pinches your temples, leaves pressure marks, and feels tighter the longer you wear it. These problems often get blamed on “my face shape,” but more often than not, it’s simply that the frame size wasn’t right to begin with.
The good news: frame size isn’t a mystery — it’s printed right on the inside of the temple arm. Once you understand those numbers, you can do a basic self-check.
The Numbers on the Inside of Your Temple Arm Are Your Frame’s ID Card
Pick up a pair of glasses and flip open the inside of the temple arm. You’ll see a marking that looks something like this:
52□18-140
These three numbers are the core dimensions of the frame, all measured in millimeters (mm). Broken down:
| Marking | Name | What It Means |
|---|---|---|
| 52 | Lens Width | The maximum horizontal width of a single lens |
| □18 | Bridge Width | The gap between the two lenses at the nose |
| 140 | Temple Length | The total length of the temple arm, from hinge to tip |
The □ symbol in the middle (sometimes a small square, sometimes shortened to a dash) marks the reference point for the bridge measurement — seeing it tells you the next number is the bridge width.
Some frames print an extra number, such as lens height, or add the brand and color code alongside. But as long as you remember the order “lens width – bridge width – temple length,” you’ve got the key information.
Quick tip: lens width plus bridge width roughly equals how far the frame spans on either side of the centerline of your nose — this is closely tied to your pupillary distance (PD). The bigger the numbers, the wider the frame.
5 Principles for Judging Fit
Once you can read the numbers, the next step is matching them to your face. Here are the five things that actually matter when choosing a frame.
1. Overall Frame Width Should Match Face Width
Looking straight on, the outer edge of the frame should ideally land right at the outline of your cheeks.
A frame narrower than your face will pinch; one much wider will slide, and visually makes your face look smaller and the frame look more obvious. The ideal is for the outer edge of the frame to sit roughly level with the widest part of your face — comfortable and natural, not tight.
2. The Frame Shouldn’t Extend Past Your Temples
Viewed from the side or straight on, the outer edge of the frame shouldn’t noticeably stick out past your temples.
If it does, the frame is too wide for you. To stay in place, the temple arms end up clamping against the sides of your head — and over time, that means sore temples and pressure above your ears.
3. The Bridge Should Sit Flush Against Your Nose
Bridge width (the number after the □) determines where the frame “hangs” on your nose.
- Plastic frames (molded bridge): the bridge width needs to match your nose bridge — too narrow pinches, too wide slides down.
- Metal frames (adjustable nose pads): the pads can be fine-tuned, giving you more margin for error, but the bridge width itself still needs to be in a reasonable range.
The bridge is where most of a pair of glasses’ weight rests. Get this wrong, and the height of the glasses is off, which throws off the optical center too.
4. Temple Length Should Match Your Ears
The temple length (the third number) should place the bend right at the groove behind your ear.
Too short, and the temple can’t reach behind your ear, so all the clamping pressure lands on top of your ear. Too long, and the tip flares out and can’t hold in place, so the glasses tend to wobble. Get the length right, and the glasses stay in place through fit — not through squeezing.
5. Frame Height Needs to Be Sufficient (Especially Important for Progressive Lenses)
Vertical lens height isn’t usually a big deal for regular frames, but if you’re getting progressive multifocal lenses, height is critical.
A progressive lens transitions from the distance zone at the top, through the intermediate zone in the middle, down to the near (reading) zone at the bottom. If the frame isn’t tall enough, the near zone gets cut off — the result is “tilting your head way up to read, and it’s still not clear.”
As a rule of thumb, progressive frames need enough lens height to fit the near zone comfortably. If you’re considering progressives, factor this into your frame choice — see our guide to progressive multifocal lenses for the full explanation.
The 3 Most Common Sizing Problems
Glasses Keep Sliding Down
There are usually two culprits: the frame is too wide overall, or the temple arms/nose pads need adjusting.
When a frame is too wide, the weight shifts forward and down, and the temples can’t hold it in place — the glasses simply slide. Switching to a properly sized frame is the most fundamental fix; if the frame size itself is fine, then the nose pads and temple bend need to be readjusted.
Frame Pinches at the Temples
This is usually because the frame is narrower than your face, or the temple arm angle is off.
With a narrow frame, the arms have to splay outward to fit, clamping against your head — wear it long enough and your temples and the area above your ears will ache. This isn’t something you “just get used to” — it’s a sizing or adjustment issue that needs to be addressed.
Frame Too Big Makes the Lenses Thicker
This one especially concerns people with high prescriptions.
The bigger the frame, the larger the lens diameter needs to be — and myopic lenses are thin in the center, thick at the edge. The bigger the frame, the thicker and heavier that edge gets. The stronger your prescription, the more pronounced the difference.
So there’s a principle for high myopia: don’t go too big on the frame, and don’t go too wide on the bridge — keeping the optical center as close to your pupil as possible lets the lens be made thinner, lighter, and better-looking. For the full strategy, see our guide to fitting glasses for high myopia.
Correct Sizing Isn’t Just About Comfort
Many people assume frame size only affects “how comfortable it feels to wear.” That’s not the whole story.
Frame size directly affects whether the lens’s optical center can align with your pupil.
Every lens has an optical center, and ideally that center should align precisely with the middle of your pupil. If the frame is too big or the bridge width is wrong, the optical center drifts away from your pupil, creating an unintended prismatic effect — at best, your vision feels a bit unstable or “floaty”; at worst, it causes dizziness and eye strain.
In other words, a properly sized frame is the precondition for your lenses to perform at their best. That’s why we don’t just ask “which one do you like” — we actually measure your face and factor the size in together with your prescription and pupillary distance.
FAQ
Q: What does the string of numbers on my glasses (like 52□18-140) mean?
That’s the frame’s size marking, in millimeters. The first number is lens width (the horizontal width of a single lens), the number after the □ is bridge width (the gap between the two lenses), and the last number is temple length (the total length of the temple arm). Together, these three numbers determine how the frame sits and sizes on your face.
Q: My glasses keep sliding down — is that a sizing problem or something that can be adjusted?
It could be either. If the frame is too wide overall, the weight shifts forward, and switching to a properly sized frame is the most fundamental fix. If the frame size itself is fine, it’s usually the nose pads and temple bend that need readjusting. We recommend bringing them into the store — we can usually tell at a glance whether it’s a sizing or an adjustment issue, and often fix it on the spot.
Q: How do I know if a frame size is right for me?
There are a few things you can check in person: whether the outer edge of the frame lines up with your cheeks, whether it extends past your temples, whether the bridge sits flush against your nose, and whether the temple bend lands right behind your ear. If you’re not sure by eye, the most accurate way is to have an optometrist measure it — factoring in your face width, nose bridge, pupillary distance, and prescription together to determine what size actually fits you.
Want to Confirm Your Frame Size Is Right? Come In and We’ll Measure It
Understanding the numbers helps you make an initial judgment, but matching them to your face and your prescription still requires an actual measurement.
Bring in the glasses you’re currently wearing, and Optometrist YoYo will measure your face width, pupillary distance, and fit in person to confirm whether your size and optical center are correct. No appointment necessary, and it doesn’t matter where your glasses were made.
- Xinzhuang store (Biyang Optometry): +886-2-2206-6700
- Banqiao store: +886-2-2253-1246
You’ll feel the difference a properly sized pair of glasses makes the moment you put them on.