Progressive Lens Fitting Height: The Millimetres a Frame Has to Hold

Three names for one vertical

The fitting height is the vertical distance from the lowest point of the lens shape to the fitting cross — the small cross engraved or printed on a progressive lens, usually a few millimetres below the pupil centre when the frame is worn. The trade uses at least three names for the same vertical: fitting height, fitting cross height, and seg height. They are not always measured from the same reference, and a number quoted without its reference is only half a measurement.

It is not the same as pupil height, although the two are close on many faces. Pupil height is where your pupil sits inside the frame; fitting height is where the lens design wants its cross. A frame can have an 18 mm pupil height and still fail a design that requires a 20 mm fitting height, because the cross is not the pupil. It is also not frame depth. Frame depth is the full vertical height of the lens opening, the B of the boxing system; fitting height is one point inside that depth, measured from the bottom.

The distinction matters because the two numbers are taken by different people at different times. The frame depth is a property of the frame and can be measured on a shelf. The fitting height is a property of the lens design plus the frame plus the wearer's posture, and it is set at dispensing. A prescription can carry a PD; it cannot carry a fitting height, because the frame is not in the prescription.

The minimum is published per design

The table below collects the minimum fitting heights published by two manufacturers for the designs in their current literature. The values are not interchangeable: a 14 mm design and an 18 mm design are both progressive lenses, and the difference between them is four millimetres of frame depth that one needs and the other does not. A frame that is deep enough for the first is not automatically deep enough for the second.

Two cautions about reading the table. Zeiss publishes a separate minimum for each fixed corridor length, which is the cleanest form of the number because it ties the height to the corridor that will sit below the cross. Hoya publishes a family minimum alongside a count of corridor lengths, but the page does not say which minimum belongs to which corridor; the table therefore reports the family minimum and says so, rather than inventing a pairing. Neither manufacturer's figure is a target. They are floors, and a lens built exactly at the floor has no margin for a frame that shifts on the nose.

The 20 mm figure in the last row is a different kind of number. It is Optometry Times' rule of thumb for a comfortable seg height, not a manufacturer's minimum, and it sits above every design minimum in the table. It is useful as a target when the frame allows it; it is not a requirement that every progressive wearer must meet.

The manufacturer figures are minima, not recommendations, and they are quoted from the pages named in the last column. Zeiss is the only one of the two that prints a separate minimum for each fixed corridor length; Hoya prints a family minimum and a corridor count, and this table does not pair them because the page does not. The 20 mm row is a clinical rule of thumb from a trade publication, and it is the only row that is not a design minimum. A lens ordered at its minimum has no allowance for a frame that settles, a pad that compresses, or a wearer who tilts the head; the number is a floor, not a fit.

The frame has to hold the minimum plus a distance zone

Above the fitting cross there has to be a clear distance zone; otherwise the top of the lens is unusable for distance vision. Optometry Times puts that minimum at 10 mm. Add it to the design minimum and you have the minimum frame depth, measured as the vertical height of the lens opening — the B of the boxing system. The arithmetic is one addition per row and it is the step that turns a lens specification into a frame specification.

The result is a little stricter than the general frame-depth advice in the same article, which puts 22 to 28 mm as the recommended depth for most general-purpose PALs. A 13 mm design plus a 10 mm distance zone needs 23 mm; a 14 mm design needs 24 mm; a 16 mm design needs 26 mm; an 18 mm design needs 28 mm; and the 20 mm rule of thumb needs 30 mm. The 22 mm bottom of the general range leaves 9 mm above a 13 mm cross, one millimetre short of the distance-zone minimum, and 8 mm above a 14 mm cross. The two published figures are not contradictory; they are accounting for different things, and the addition is the stricter of the two.

This is also the arithmetic behind short-corridor designs. A shallow frame cannot hold a long corridor and a distance zone at the same time. The manufacturer's answer is a design with a lower minimum, not a lower fitting cross: the cross has to sit where the design says, and the way to fit a 24 mm frame is to choose a 14 mm design rather than to squeeze an 18 mm one into it. The table's 30 mm row is the same point at the other end: a 20 mm seg height is comfortable, and it needs a frame most fashion frames do not have.

The addition is this page's arithmetic on two published figures: the design minimum in the first column and the 10 mm distance zone in the second. The 22 to 28 mm range is Optometry Times' general recommendation for most general-purpose PALs, not a rule derived from the same addition; that is why the 22 mm bottom can sit one millimetre below the 23 mm result for a 13 mm design. The comparison column should be read as a check on the arithmetic, not as a claim that one source contradicts the other. Frame depth is the vertical height of the lens opening at its tallest, not the diagonal and not the frame's overall height.

How to measure it on your own frame

You need two numbers and one subtraction. First, the frame's B: the vertical height of one lens opening at its tallest point, measured inside the rim with a ruler. Do not use the diagonal or the frame's overall height; both are larger and both will make a shallow frame look deeper than it is. Second, the height of your pupil centre in the same lens opening, with the frame adjusted on your face and your gaze on a distant target at eye level. Measure from the bottom inside edge of the rim to the centre of the pupil, in millimetres.

Now compare the pupil height with the design minimum for the lens you are considering, and compare the frame's B with the minimum plus 10 mm. A worked pair: a frame with a 26 mm B and a 16 mm pupil height has 10 mm above the pupil, exactly the distance zone. It can hold a 16 mm design with no margin and an 18 mm design not at all. A frame with a 24 mm B and a 16 mm pupil height has 8 mm above the pupil, two short of the distance zone; the fix is a 14 mm design, which leaves 10 mm, not a lower cross on the 16 mm design.

The same arithmetic explains the 22 mm case that looks as if it should work. A 22 mm frame with a 13 mm pupil height leaves 9 mm above the pupil, one millimetre short of the 10 mm distance zone. It is inside the general 22 to 28 mm range and still cannot hold the shortest design in the table with the full distance zone. If the frame is a favourite and the number is that close, the honest options are a different lens design, a different frame, or an acceptance that the distance zone is 9 mm rather than 10 — not a fitting cross moved below the design's floor.

Measure both eyes. A frame that sits asymmetrically can give two different pupil heights, and the lower one is the one that decides whether the design fits. If the difference is more than a millimetre, have the frame adjusted before deciding; a pad that has settled can move the number more than a millimetre without anyone noticing.

What the corridor length does to the reading area

The fitting height is the top of the corridor's budget, not the whole of it. A progressive lens divides its vertical height among three zones: distance above the cross, intermediate through the corridor, and near below it. A 2004 optical analysis of 28 PALs found that the three viewing-zone areas trade off against each other, with negative correlations between them, and that the power rate in the corridor is closely related to the width of the intermediate and near zones. In plain terms: a longer corridor spends height that the near zone could have used, and a shorter corridor buys near area at the cost of a faster power change.

This is why the manufacturer's minimum is tied to a corridor length. Zeiss prints 14 mm for the short corridor, 16 mm for the medium and 18 mm for the long, and those three numbers are the same three corridor lengths. Hoya prints a family minimum and a count of corridor lengths without pairing them, which leaves the dispenser to use the design's fitting chart rather than the family number alone. The table in this page is a starting point for the frame conversation, not a substitute for the lens manufacturer's fitting chart.

The practical consequence is that a frame at the exact minimum is a frame with no room for a longer corridor. If the wearer wants a wider reading area, the frame has to be deeper or the corridor shorter; there is no third option that keeps both. That is the trade the 2004 analysis measured, and it is the reason a 24 mm frame and an 18 mm design cannot be made to behave like a 28 mm frame and a 16 mm design by moving the cross.

Why the number is not on the prescription

A prescription describes the power the eye needs. It does not describe the frame the lens will sit in, the height of the pupil in that frame, or the corridor length the wearer's reading habits call for. Those are dispensing measurements, taken with the frame on the face, and they change when the frame changes. Two pairs of glasses with the same prescription can have different fitting heights because they have different frames.

That is also why a home measurement is a check rather than a substitute. You can measure your pupil height in a frame you already own, and you can measure the frame's B, and you can run the subtraction. What you cannot do at home is choose the design's corridor length, confirm that the frame's wrap and pantoscopic tilt match the manufacturer's assumptions, or verify that the lens will be edged to the cross the way the design expects. Those are the dispenser's part of the job, and the arithmetic here is the part you can bring to the conversation.

The two numbers work together. The horizontal one — your PD — tells the lab where the optical centre sits across the lens. The vertical one — the fitting height — tells it where the cross sits up the lens. A pair of progressives needs both, and a page about one is not a page about the other.

The record, in the order it arrived

The published record behind this page is short and it is recent at one end: a 2004 optical analysis of how PAL zones trade off, a 2017 clinical article that supplies the 10 mm distance zone and the 22 to 28 mm frame range, and two manufacturer pages that publish the minima this page turns into frame depths.

The first two rows are a peer-reviewed paper and a clinical trade article; the third and fourth are manufacturer literature, which is the only kind of source that can publish a design minimum. The manufacturer dates are the revision or access dates printed on the documents, not the date the design was introduced. The 20 mm rule of thumb is not a manufacturer minimum and is not used as one in this page's arithmetic; the six frame depths use the 13 to 18 mm design minima plus the 10 mm distance zone, and the 20 mm row is shown separately because the article recommends it as a comfortable target.

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