Segment height (seg height) is the vertical distance from the bottom edge of your lens to a specific point on your eye, and it tells the lab where to place the different vision zones in multifocal lenses. Getting this measurement wrong by even a millimeter or two can mean blurry reading vision, neck strain, or a pair of progressives that never feels right. The process is straightforward once you know which landmark to measure to, and that depends on whether you’re fitting bifocals, trifocals, or progressive lenses.
Why Seg Height Matters
Seg height determines where the transition between your distance and near vision zones sits in your lenses. In progressive lenses, it positions the corridor that blends distance, intermediate (computer), and reading zones. In bifocals, it sets the visible line separating your distance and reading areas. If the seg height is too high, you’ll look through the reading portion when you’re trying to see across the room. Too low, and you’ll have to tilt your chin up uncomfortably to read.
This measurement is only needed for multifocal lenses. Single vision lenses have one focal zone across the entire surface, so there’s no segment to position.
The Landmark Changes by Lens Type
The point you measure to on the eye is different for each type of multifocal:
- Progressive lenses: Measure to the center of the pupil. This is where the fitting cross of the lens will align.
- Bifocals: Measure to the top of the lower eyelid. The visible bifocal line sits at this level so it stays out of your distance vision but catches your reading gaze naturally.
- Trifocals: Measure to the bottom edge of the pupil. The top of the intermediate segment sits slightly higher than a bifocal line would, giving the middle zone enough room to be useful.
As a general rule, bifocal and trifocal segments land about 1 to 3 millimeters below the base of the pupil, with bifocals sitting slightly lower and trifocals slightly higher within that range.
Adjust the Frame First
Seg height is only accurate if the frame is sitting on your face the way you’ll actually wear it. Before any marking or measuring happens, the frame needs to be properly adjusted. The bridge should rest comfortably on your nose without sliding, and the temples should hold the frame level without pinching. If the frame tilts, shifts, or sits too high or low during measurement, your seg height will be off once you start wearing the finished glasses.
For higher prescriptions, vertex distance (the gap between the back of the lens and the surface of your eye) also matters, because changes in that spacing can shift the effective power of the lens. The frame should be positioned at the same distance a final pair would sit.
Step-by-Step Measurement
You’ll need the frame the lenses will go into, a non-permanent marking pen (a fine-tip dry erase marker works), and a millimeter ruler or pupil height gauge.
Marking the Reference Point
Put the adjusted frame on and look straight ahead at a point in the distance. A second person should sit or stand directly in front of you with their eyes at the same level as yours. This matters because looking up or down at the person marking changes where the pupil appears in the frame.
While you look straight ahead, the helper uses the marking pen to place a small dot on the demo lens (or a piece of clear tape over the lens opening) at the appropriate landmark: center of the pupil for progressives, top of the lower lid for bifocals, or bottom of the pupil for trifocals. Do this for each eye separately, since many people’s eyes sit at slightly different heights relative to the frame.
Drawing a Horizontal Line
Remove the glasses carefully without smudging the dot. Draw a short horizontal line, roughly an inch long, through each dot. This line makes it easier to measure precisely and to verify the mark later. A helpful technique: rest the pen flat against the edge of a table, press the dot against the pen tip, then slide the frame left and right. The table edge keeps the line perfectly horizontal.
Measuring the Distance
Hold the ruler vertically against the lens with the zero end at the very bottom of the lens opening (the lowest point where the lens will sit in the frame). Measure straight up to the horizontal line you drew. That distance, in millimeters, is your seg height. Most seg heights for progressive lenses fall somewhere between 14 and 22 mm depending on the frame size and how it sits on your face.
Verifying the Mark
Put the glasses back on and stand up. Have someone check the mark from the side. For progressive lenses, your line of sight should pass directly through the horizontal line when you look straight ahead. For bifocals and trifocals, your line of sight should be just above it.
For bifocals and trifocals, there’s a simple tape test: place a strip of clear tape along the horizontal line. When you look straight ahead or walk around, the tape should be completely out of your line of sight. When you look down to read, it should be directly in the way. If it interferes with distance vision, the seg is too high. If you can read comfortably without looking through the tape, the seg is too low.
Seg Height vs. Frame B Measurement
You’ll sometimes see seg height discussed alongside the frame’s “B measurement,” which is the total vertical height of the lens opening. These are related but not the same thing. Seg height is measured from the bottom of the lens to your fitting point. The B measurement is simply the frame dimension. Opticians sometimes calculate a value called “segment top position” by subtracting the seg height from the B measurement, which tells them how far above or below the horizontal center of the frame the segment sits. You don’t need to calculate this yourself when ordering glasses, but if a form asks for both seg height and B measurement, measure the vertical height of the lens opening at its tallest point for B.
Common Sources of Error
Most seg height mistakes come from a few predictable places. The frame sliding down the nose during measurement is the most common culprit, because even 2 mm of slippage translates directly into 2 mm of error. Measuring with your eyes at a different height than the person wearing the glasses creates parallax, making the pupil appear higher or lower than it really is. And measuring from the wrong starting point (the bottom of the frame rim instead of the bottom of the lens groove, for instance) can throw things off by a millimeter or two.
If you’re measuring at home for an online order, take the measurement two or three times and compare. Consistency within 1 mm across attempts is a good sign you’ve got it right. If your results vary by more than that, check that the frame isn’t shifting between attempts and that you’re keeping your gaze level.
Measuring for Each Eye Separately
Many people have slight facial asymmetry, meaning one eye sits a bit higher in the frame than the other. For progressive lenses especially, measuring each eye independently (called monocular seg heights or “OC heights”) gives the lab the information it needs to position each lens corridor precisely. When you record your measurements, label them for the right eye and left eye separately rather than using a single number for both.