Buying glasses online means doing two things a fitting room usually does for you: choosing a frame that actually fits your face, and — if you're getting progressive lenses — making sure the lens is set up correctly for exactly where your eyes sit in the frame. Neither is complicated once you know what the numbers mean. Here's the full walkthrough.
Part 1: A Quick Recap on Frame Size
Choosing a well-fitting frame comes down to comparing four numbers — lens width, bridge width, temple length, and lens height — against a pair you already own. We cover this in full, with diagrams showing exactly where each measurement is printed and where it falls on the frame, in our companion guide: Glasses Size Guide: Understanding Your Frame Measurements.
The short version: match your current frame's lens width and bridge width within about 1–2mm for a comfortable fit, and check the lens height (B-measurement) specifically if you're ordering progressives — that's the number this guide picks up next.
Part 2: Segment Height for Progressive Lenses
This section applies only if you're ordering progressive (no-line multifocal) lenses. If you're ordering single-vision lenses, you can skip this entirely — segment height isn't used for single-vision prescriptions.
What Segment Height Actually Is
Segment height is the vertical distance, in millimeters, from the bottom edge of the lens (as it sits in your chosen frame) up to the center of your pupil. This measurement tells your lab exactly where to position the transition between your distance vision (top of the lens) and your near vision (bottom of the lens) so both zones line up with where your eyes actually look through the lens.
An important detail: segment height is specific to the combination of your face and your chosen frame — not a fixed personal number. The same person can have a different segment height in a tall, rectangular frame versus a small, round one. This is also why your eye doctor's original prescription usually won't include it: segment height is a dispensing measurement tied to the specific frame you choose, not part of your refraction.
Here's what that measurement is actually positioning inside the lens itself — the three vision zones a progressive lens blends together, and where segment height marks the reference point between them:
How segment height positions the distance, intermediate, and near zones
A progressive lens isn't divided into hard, sharp-edged sections — the three zones blend smoothly into each other. But the general layout holds: distance vision (driving, watching TV, general use) sits above the fitting cross where your pupil rests; near vision (reading, close work) sits well below it; and a narrower intermediate zone (computer distance, arm's length) connects the two in between. Segment height determines exactly where that fitting cross falls in the frame — get it meaningfully wrong, and all three zones shift out of position relative to where your eyes actually look.
Three Ways to Provide Your Segment Height
How Aldermiller Captures Your Segment Height
A quick note on process first: Aldermiller does not ship frames to customers for local in-person measurement before an order is placed. Every order — including the frame — is fabricated and shipped as one complete pair, on our standard timeline. If precise, professionally-measured segment height matters a lot to you, the better path is Option 2 below, using a frame you already own, rather than waiting to receive an unfabricated Aldermiller frame first.
Option 1 — From Your Current Progressive Glasses
If you already wear progressives and have a recent receipt, dispensing slip, or record from your last optometrist or optical shop, your segment height may already be written down — ask your previous provider if you're not sure.
How you use that old number depends on how similar your new Aldermiller frame is in size to your old one:
● If the new frame's lens height (the vertical B-measurement, listed on every Aldermiller product page) is within about 2–3mm of your old frame's lens height, you can enter your old segment height number directly — the difference is small enough not to matter.
● If the new frame's lens height is noticeably different from your old one — for example, moving from a tall rectangular frame to a smaller, rounder one — a direct copy becomes less reliable. In that case, a short adjustment gets you a meaningfully more accurate number:
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If you know your OLD frame's lens height (B-measurement) as well as your old segment height: 1. Distance from top = Old lens height − Old segment height 2. New segment height = New frame's lens height − Distance from top Example: old lens height 32mm, old segment height 18mm → distance from top = 14mm. New Aldermiller frame lens height is 30mm → new segment height = 30 − 14 = 16mm. |
This works better than a direct copy because the distance from the top of the lens tracks closer to your actual eye position — the part of the fit that's genuinely stable across different frames — while the bottom edge of the lens (what a direct copy relies on) shifts around more between frame shapes.
Don't have your old frame's lens height handy? A straight copy of your old segment height number is still a reasonable choice if the frames look similarly sized — just less precise than the adjusted version above.
One note on this option and Option 2 below: since these use a number you're providing, if the fit doesn't feel right after adaptation, that falls under our standard 30-day Return Policy rather than a free adjustment — see our Prescription Policy for details. That's different from Option 3 below, which carries its own one-time free adjustment specifically because Aldermiller chose the method rather than you.
Option 2 — Measure It Yourself at Home
Since Aldermiller ships fabricated pairs rather than sending a frame ahead of your order, this measurement uses a frame you already own — not the new Aldermiller frame itself. Any glasses you currently have on hand work, including non-prescription readers or sunglasses; it's the frame's physical size that matters here, not what's in the lenses. The closer that frame's dimensions are to the new Aldermiller frame (compare using the lens width and lens height numbers from Part 1), the more reliable this measurement will be.
This takes about five minutes and works well with a helper, though it's possible alone with a mirror.
● Put on the frame you're using for this measurement, and make sure it's sitting the way you'd normally wear glasses.
● Look straight ahead at a distant point, keeping your head level and your posture natural — not tilted up or down.
● Have a helper mark the center of your pupil directly on the lens (or the frame's demo lens) with a fine washable marker, while you continue looking straight ahead.
● Using a millimeter ruler, measure straight up from the bottom inner edge of the lens opening to that mark. That number is your segment height.
● Repeat once to confirm — if your two measurements differ by more than 1mm, take a third and use the average, or try again with a helper if you measured alone.
Option 3 — Let Aldermiller Estimate It for You
If you're not able to measure at home and don't have a number from a previous provider, we can provide a best-estimate segment height using an industry-standard formula: 48% of the lens's vertical height (technically called the B-measurement) in your chosen frame, measured from the bottom of the lens opening. This is the same general approach used by many online eyewear retailers when a customer doesn't provide their own measurement.
Worked example: a frame with a 48mm B-measurement would get an estimated segment height of 48% × 48mm = 23mm (rounded to the nearest millimeter, matching how labs typically work).
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Important: this estimate is a reasonable average, not a personal measurement. It's calculated from the frame's dimensions, not from where your eyes actually sit when you wear it — for most wearers and most frame shapes it lands close enough to work well, but it is inherently less precise than a measurement taken on your own face in your own frame. If your eyes sit noticeably higher or lower than average within a frame shape, the estimate is more likely to be off. |
A few honest notes on what this means in practice:
● Modern progressive lens designs are meaningfully more forgiving of small segment height differences than older designs — a few millimeters off usually means a slightly smaller usable near-vision zone, not lenses that don't work at all.
● If you choose the estimate and find your progressives don't feel right after the normal 1–4 week adaptation period, we'll re-edge your lenses to an adjusted segment height once, at no charge, provided you contact us within 60 days of delivery — email hello@aldermiller.com with your order number. This one-time accommodation applies specifically to orders using our formula estimate; see our Prescription Policy, Section 10, for full details.
● If precision matters a lot to you — for example, if you've had a difficult time adapting to progressives in the past — we'd genuinely recommend Option 1 or 2 over the estimate if either is available to you.
Quick Reference: Which Option Should You Use?
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Your Situation |
Recommended Option |
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You have a recent progressive record AND your new frame is similar in size (within ~2–3mm) to your old one |
Option 1 — use that number directly |
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You have a recent progressive record but your new frame is a noticeably different size |
Option 1 — use the adjustment formula |
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You're willing to spend 5 minutes with a helper and a ruler |
Option 2 — most accurate self-measurement |
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You want the simplest checkout experience and are comfortable with an estimate |
Option 3 — Aldermiller's 48% formula estimate |
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You've struggled with progressive adaptation before |
Option 1 (adjusted) or Option 2 strongly recommended over Option 3 |
Questions about any of this? Email us at hello@aldermiller.com — we're happy to walk through your specific frame and measurements before you place your order.