2026-03-11
You're Probably Wearing the Wrong Size Glasses
Here's something most people never think about: glasses come in sizes. Not just small, medium, and large — specific millimetre measurements that determine whether a frame actually fits your face or just sort of sits on it. And the wrong size doesn't just look off. It can affect your vision.
If you've ever felt like your glasses slide down constantly, pinch behind your ears, or just look slightly "wrong" in photos, the issue probably isn't the style. It's the fit.
The Three Numbers That Matter
Look at the inside of your glasses temple (the arm that goes over your ear). You'll usually find three numbers printed or engraved there, something like 52-18-140. These aren't random — they're the frame's measurements in millimetres:
- Lens width (52mm): The horizontal width of each lens. This is the most important number for fit. Too wide and your eyes won't sit at the optical centre. Too narrow and the frame will squeeze.
- Bridge width (18mm): The distance between the two lenses — the part that sits on your nose. Too narrow and the frame pinches. Too wide and it slides down.
- Temple length (140mm): The length of each arm from the hinge to the tip that hooks behind your ear. Most adults need 135mm to 145mm.
If you have a pair that fits well right now, write these numbers down. They're your starting point for any new frames.
Why Optical Centre Matters More Than You'd Think
Your lenses are ground so the sharpest, most accurate part of the prescription sits directly in front of your pupils. This is called the optical centre. When a frame is too wide or too narrow for your face, your pupils don't align with the optical centre of the lens.
What does that mean in practice? You might not notice anything dramatic, but you could experience:
- Mild eye strain that builds through the day, especially with computer work
- A slight pulling sensation, like your eyes are working harder than they should
- Headaches that seem unrelated but disappear when you take your glasses off
This is particularly noticeable with stronger prescriptions and progressive lenses, where the sweet spot for each viewing zone is smaller.
How to Tell If Your Current Glasses Fit Properly
Stand in front of a mirror and check these things:
Your eyes should sit near the centre of each lens
If your pupils are pushed toward the inner edge or sitting noticeably off-centre, the lens width is wrong. A little variation is normal, but if you can see a lot more frame on one side of your eye than the other, that's a sizing issue.
The frame should follow your brow line
The top of the frame should roughly follow the natural curve of your eyebrows. Frames that sit well above your brows make your face look bottom-heavy. Frames that cut across or below them look like they're sliding down even when they're not.
The bridge should sit flush on your nose
Gaps between the bridge and your nose mean the bridge is too wide. Red marks or pinching mean it's too narrow. Adjustable nose pads give some flexibility here, but they can't fix a bridge that's fundamentally the wrong size.
The temples shouldn't press into your head
The arms should follow the contour of your head with light, even contact. If they dig in above your ears, the frame is too narrow overall. If they stick out at an angle before curving back, the frame is too wide.
Common Fit Problems and What They Actually Mean
"My glasses always slide down"
This is almost always a bridge issue. The bridge width is too large for your nose, or the nose pads need adjustment. Less commonly, the temples are too long and aren't gripping behind your ears effectively. This is rarely fixed by pushing your glasses up repeatedly — get the bridge adjusted or try a different bridge size.
"My glasses leave red marks on my nose"
The frame is too heavy for the nose pad contact area, or the pads themselves need repositioning. Lighter frame materials (titanium, certain acetates) and properly adjusted nose pads usually solve this. Some people do better with frames that have a saddle bridge (one continuous piece across the nose) rather than individual nose pads.
"My glasses feel tight by the end of the day"
The frame is too narrow. Your head expands slightly throughout the day due to normal fluid changes and muscle activity. A properly fitted frame accounts for this. If your glasses feel perfect in the morning but uncomfortable by evening, they're borderline too tight.
Online Sizing vs. In-Person Fitting
Buying glasses online has become popular, and the sizing tools have improved. Most online retailers let you filter by frame measurements and offer virtual try-on features. These work reasonably well for people who already know their measurements and have a straightforward prescription.
But there are things a screen can't replicate. How a frame balances on your specific nose shape. Whether the temple curve matches the contour behind your ears. How the frame's weight distributes across your face. For progressive lenses especially, precise fitting measurements — pupillary distance, segment height, pantoscopic tilt — need to be right for the lenses to work properly.
If you know your sizes and you're buying a simple single-vision pair, online can work fine. For anything more complex, or if you've never found a frame that feels really right, in-person fitting makes a genuine difference.
Finding Your Fit
Next time you're looking at frames, start with the numbers rather than the style. Find your lens width, bridge width, and temple length from a pair that fits well, and use those as your baseline. A frame that fits properly looks better, feels better, and lets your lenses do their job the way they're supposed to.
Not sure what your measurements should be? Come in and we'll measure you properly — it takes about two minutes and makes every future frame choice easier.