The B measurement on glasses is the vertical height of the lens, measured from the top edge to the bottom edge. It tells you how tall the lens opening is in millimeters and plays a key role in determining whether a frame can accommodate certain lens types, especially progressives and bifocals.
How the B Measurement Works
Eyeglass frames are sized using a standardized system called the Boxing System, which draws an imaginary rectangle around each lens. The B measurement is the vertical side of that rectangle: the distance between two horizontal lines touching the very top and very bottom edges of the lens. A frame with a B measurement of 35 mm, for example, has a lens opening that is 35 mm tall.
This measurement sits alongside several other standard dimensions:
- A measurement: The horizontal width of the lens.
- DBL (distance between lenses): The gap between the two lenses, sitting over your nose bridge. Combined with the A measurement, it determines how wide the entire frame front is.
- ED (effective diameter): The longest diagonal measurement needed to cut a lens for the frame. The A and B values together serve as the basis for calculating this number.
Together, these dimensions give lens manufacturers the exact specifications they need to cut and fit lenses into your frame.
Where to Find It on Your Glasses
Frame measurements are usually printed on the inside of the temple arm (the piece that goes over your ear). You’ll typically see a series of numbers separated by dashes or small symbols, like “52-18-140.” These represent the A measurement, the DBL, and the temple length. The B measurement is not always printed on the frame itself, which is why many people don’t encounter it until they’re shopping online or getting fitted for progressive lenses.
If you need to measure it yourself, place a ruler vertically across the lens at its tallest point. Measure from the very top edge to the very bottom edge. If the frame has a beveled rim (the groove that holds the lens in place), include the depth of that bevel in your measurement, since the actual lens extends into it.
Why It Matters for Progressive Lenses
The B measurement becomes critical when you need progressive lenses. Progressives pack three vision zones into a single lens: distance at the top, intermediate in the middle, and reading at the bottom. These zones are arranged along a vertical corridor, so the lens needs enough height to fit all three comfortably.
Most progressive lens designs require a minimum fitting height, which is the distance from your pupil center down to the bottom of the lens. The reading area alone should be at least 4 to 5 mm tall to be usable. On top of that, optical professionals generally recommend choosing a frame that adds 2 to 4 mm beyond the manufacturer’s stated minimum fitting height. This extra room keeps you from losing half the reading zone to a frame that’s too shallow.
In practical terms, this means frames with a B measurement under about 28 mm can be difficult or impossible to fit with standard progressive lenses. Short, narrow frames look sleek, but they compress the corridor and shrink the reading area. If you wear progressives or expect to in the future, paying attention to the B measurement before you buy can save you from ending up with lenses that feel cramped or hard to use.
How B Measurement Affects Lens Shape and Fit
The ratio between the A and B measurements defines the overall shape of your lenses. A frame where the A measurement is much larger than the B will look wide and rectangular. When the two values are closer together, the lens appears rounder or more square. This is useful to know when shopping online, where you can’t try frames on. Comparing the A and B numbers gives you a reliable sense of the lens shape before anything arrives at your door.
A taller B measurement also means more lens material, which can matter if you have a strong prescription. Thicker lenses weigh more, and that extra weight is multiplied by a larger lens area. For high prescriptions, a moderately sized B measurement (rather than an oversized one) helps keep the lenses thinner and lighter.
Conversely, a very small B measurement limits your field of view vertically. If you spend a lot of time looking down at a desk or up at overhead monitors, a taller lens gives you more usable vision without tilting your head as much. The right B measurement depends on your prescription, your lens type, and how you actually use your glasses throughout the day.