Every pair of glasses has three numbers printed on the inside of the frame, usually on the temple arm. These numbers, measured in millimeters, tell you the lens width, bridge width, and temple length. Finding your size means matching these three measurements to your face so the frames sit comfortably, look proportional, and (if you have a prescription) position the lenses correctly in front of your eyes.
The Three Numbers on Every Frame
Look at the inside of your current glasses or any pair in a store. You’ll see something like 52-18-140. Here’s what each number means:
- Lens width (first number): The horizontal width of each lens, not including the frame around it. Typical range is 40 to 60 mm. This is the single most important number for overall fit.
- Bridge width (second number): The shortest distance between the two lenses, which corresponds to the part of the frame resting on your nose. Typical range is 14 to 24 mm.
- Temple length (third number): The length of each arm from the hinge screw to the tip that curves behind your ear. Common sizes are 135, 140, 145, and 150 mm.
If you already own a pair that fits well, write down these three numbers. They’re the fastest shortcut to finding your next pair.
How to Measure Your Face
If you don’t have a well-fitting pair to reference, start with your total face width. Hold a ruler horizontally across your face, just below your eyes, and measure the distance from one temple to the other. This temple-to-temple measurement tells you how wide your frames should be overall.
That face width maps to a lens width like this:
- Narrow face (under 129 mm): Lens width of 50 mm or smaller
- Medium face (130 to 139 mm): Lens width of 51 to 55 mm
- Wide face (over 139 mm): Lens width of 56 mm or larger
These are starting points. A frame that’s 2 mm wider or narrower than your “ideal” can still fit perfectly well, so treat them as guidelines rather than hard cutoffs.
Getting the Bridge Width Right
The bridge width affects comfort more than any other measurement. A bridge that’s too narrow will pinch your nose. One that’s too wide lets the glasses slide down constantly.
If you have a narrower nose or close-set eyes, look for bridge widths on the lower end, around 14 to 18 mm. A wider nose or wide-set eyes generally calls for 19 to 24 mm. Nose height matters too. If your nose sits higher on your face, frames with a bridge that aligns with your brow line tend to work best. If your nose is lower, frames with a curved brow line and a lower bridge position feel more natural.
Choosing the Right Temple Length
Temple arms need to be long enough to reach behind your ears without sticking out far past them. Most adults fall somewhere between 135 and 145 mm. You can check your current fit by looking at yourself from the side: the arm should curve gently behind your ear with no more than a small amount of extra length beyond it. If the arms press tightly against the sides of your head or leave red marks, the temples may be too short. If they stick out noticeably past your ears or feel loose, they’re too long.
Why Pupillary Distance Matters
If you’re ordering prescription glasses, you’ll also need your pupillary distance, or PD. This is the distance between the centers of your pupils, and it determines where the optical center of each lens is placed. The average adult PD is about 63 mm, but the normal range spans from 50 to 70 mm.
An incorrect PD measurement can cause eyestrain, headaches, and distorted vision because the strongest part of the lens won’t line up with your pupils. Your eye doctor may include PD on your prescription, but not all do. You can measure it yourself by holding a millimeter ruler across the bridge of your nose while looking straight into a mirror, marking the center of each pupil. It helps to close one eye at a time to get an accurate reading for each side.
One detail worth knowing: your PD shrinks by about 3 mm when you focus on something up close. Some retailers ask for both a “distance PD” and a “near PD” for this reason, especially if you’re ordering reading glasses or progressives.
Extra Height for Progressive Lenses
If your prescription includes progressive (no-line bifocal) lenses, frame height becomes a factor that single-vision wearers don’t need to worry about. Progressive lenses pack three zones of vision into one lens: distance at the top, intermediate in the middle, and reading at the bottom. Each zone needs enough vertical space to be usable.
The reading area alone needs a minimum of 4 to 5 mm of height, and the full fitting height (from where your pupil sits down to the bottom of the lens) should allow an additional 2 to 4 mm on top of the manufacturer’s minimum. In practice, this means frames that are too shallow vertically will cut off part of your reading zone, making the lenses frustrating to use. When shopping for progressives, look for frames with a taller lens opening and ask the retailer about the minimum fitting height for the specific lens design they recommend.
Frame Width and Face Shape
Size and shape work together. A frame can be the right width for your face but still look off if the shape clashes with your features. The general principle is contrast: angular frames balance soft, round features, while curved frames soften strong angles.
- Round face: Rectangular and geometric frames add definition.
- Square face: Round and oval frames soften angular jawlines.
- Heart-shaped face: Square and aviator styles balance a wider forehead and narrower chin.
- Diamond face: Round, oval, and cat-eye shapes complement prominent cheekbones.
- Oval face: Proportionally balanced, so most frame styles work.
Regardless of face shape, the frames should be roughly as wide as the widest part of your face. Frames noticeably wider than your face look oversized, while frames narrower than your cheekbones can make your face appear wider by contrast.
Putting It All Together
Start with the pair you already own if one fits comfortably, and note its three numbers. If you’re starting from scratch, measure your face width temple to temple to find your lens width range, then match bridge width to your nose and temple length to your ear placement. For prescription orders, get your PD measured accurately. If you wear progressives, prioritize frames with enough vertical height to accommodate all three vision zones. And when you’re comparing two sizes that are close, err toward the slightly larger frame. A skilled optician can adjust nose pads and temple tips to tighten the fit, but there’s no fix for a frame that’s fundamentally too small.