glasses frame size & fit calculator

how the numbers are derived.

the short version, then the full method underneath for anyone who wants to check our working.

the short version

a photo has no ruler in it. to turn what the camera sees into millimetres, we need something in the picture whose real size we already know. we use your iris - the coloured ring around your pupil - because it is always there, it is round, and it does not change with your expression or your age. we take it as 11.7mm across, which is the usual figure for an adult.

that is an assumption, and it is the weak point. irises vary by about half a millimetre between people. that sounds like nothing, but everything else is scaled from it, so half a millimetre of iris is about six millimetres of face. taking more readings does not help, because the error is the same in every one.

a bank card fixes it. every card is exactly 85.60mm wide, by international standard. hold one against your forehead, mark its edges, and we can measure your actual iris instead of assuming it. that turns the whole reading from an estimate into a measurement, and it is the single best thing you can do for accuracy here.

we correct for the shape of a head. the face tracker marks points on your outline as the camera sees it, and those sit slightly inside where an optician would put a ruler, because a head is round and a photo is flat. each width carries a small correction for that. those corrections were worked out against one person's measurements, so treat them as good to a few millimetres rather than exact.

the fit window comes from your temple track - the width of your head where the arms rest. a frame front sits inboard of that and the arms flex outward to reach your ears, so the range we look for is your temple track minus 4mm to minus 20mm.

what we will not tell you. where the lens centres should sit vertically, or how far down the lens your pupil lands. those depend on how a particular frame sits on your particular nose, and no photograph taken from the front can know that. an optician measures it with the real frame on your face. this is sizing, not dispensing.

the full method

every constant below is quoted from the code that runs above, and every assumption is named as one.

the scale: why 11.7mm

a photograph has no units. before a pixel can become a millimetre, something of known size has to be in the picture. the iris is the only thing that qualifies: it is always there, it is round, and it does not change size with your expression or your age. we take it as 11.7mm across, the figure most commonly cited for adults, and scale everything else from that:

millimetres per pixel = 11.7 ÷ iris width in pixels

the catch: adult irises vary by about half a millimetre either way, which is roughly 4%. on a 156mm face that is 6mm. and it is a steady bias rather than random noise, so taking more readings cannot average it out.

that is why the bank card option exists. every bank card in the world is 85.60mm wide - it is fixed by a standard, iso 7810. mark its two edges and the assumption is replaced by a measurement of your own eye:

your iris = iris width in pixels × (85.60 ÷ card width in pixels)

the calculator accepts a result between 10.2 and 13.4mm and refuses anything outside, on the grounds that a figure beyond the human range means the marks are not on the card.

the correction factors: 1.07, 1.07, 1.10

the face tracker returns points on your outline as the camera sees it. those are not the points a dispenser would put a ruler on, so each width carries a correction:

face width = pixel width × scale × 1.07
temple track = pixel width × scale × 1.07
bridge = pixel width × scale × 1.10

the three differ because the mesh sits differently against each one. across the nose it reads high and the correction is doing two jobs at once - moving from the mesh's span to the nose, then from the nose to the frame bridge that clears it, which is always wider. across the face and the temples it reads low, because the face-oval contour sits inside the true head silhouette - so those two carry the same correction upward.

these were fitted to one person, against physical objects. temple track against the arms of a frame worn comfortably at 153mm and a ruler above the ears at 155mm. bridge against that frame's 21mm bridge, confirmed by a second frame at 22mm. face width is the same span again: the field feeds the temple track when that is left on “same as face”, so both are temple-to-temple measurements and both carry the same factor. it is not a cheekbone measurement, whatever the name suggests. an entered pupillary distance overrides the scale they all depend on, because a figure off a prescription beats any factor.

depth correction and the 400mm reference

hold the phone closer and the effect above gets stronger, so the same face measures narrower. we used to correct for that by scaling every reading back to a 400mm reference, using the distance the face tracker reports.

that correction is currently switched off. the number the tracker gives is not a distance. it solves the head pose against an assumed camera and its own canonical head, so it carries the ratio between the assumed lens and the real one. on laptop webcams it lands close to the truth; on a phone front camera, which is much wider, it reads roughly 1.6 times high - 52cm at a real 32cm. no browser exposes a focal length or a field of view, so it cannot be put right.

applying a correction with a wrong distance is worse than applying none, so we apply none. the practical effect: hold the camera at arm's length and the reading is unaffected either way, because that is where the correction was doing nothing. hold it much closer and your widths will read a little narrow, much further and a little wide - about 2% at 30cm and 50cm respectively. an entered pupillary distance removes even that, because it re-scales everything to a figure that does not depend on distance at all.

how the pupils are found

the face is tracked with google's mediapipe face landmarker, which returns 478 points on every frame. the first 468 map the face. the last ten come from a separate pass that finds the irises specifically, and those ten are the ones that matter here. 468 is the right iris centre and 469-472 its boundary; 473 is the left centre and 474-477 its boundary. pupillary distance is measured centre to centre. iris width is twice the average distance from a centre out to its four edge points. averaging four points rather than trusting one is what keeps the ruler steady from frame to frame.

pd = distance(468, 473) × scale
iris width = 2 × mean distance from centre to its four boundary points

the widths use named mesh points rather than the silhouette: 234 and 454 for the cheekbones, 127 and 356 for the temples, 245 and 465 for the nose at the bridge.

why the crosshairs never move

this one is worth stating plainly, because it looks like the opposite of what happens. the two crosshairs are drawn at fixed positions - one either side of centre, separated by exactly the pupillary distance at the diagram's millimetre scale. they are not tracking your eyes. the photograph is moved to them.

on capture, the detected pupils give a separation in image pixels and an angle. the photo is then scaled so that separation equals the pupillary distance in diagram units, rotated so the pupil line is level, and translated so the pupil midpoint lands on the crosshair midpoint.

photo scale = (pd × diagram scale) ÷ pupil separation in pixels
photo rotation = −atan2(Δy, Δx) of the pupil line

that ordering is deliberate. the diagram is drawn in millimetres and is the reference; the photograph is the thing fitted to it. it is why a frame drawn over your face is at true scale rather than approximately sized, and why dragging the photo afterwards cannot corrupt the measurements - it moves the picture, not the numbers.

the fit window: minus 20 to minus 4

frame width is measured across the outer edges of the endpieces and is the dimension that decides whether a frame sits or splays. it is not printed on the frame. the window is taken from the temple track:

window = temple track − 20mm to temple track − 4mm

the front sits inboard of the head's widest point and the arms open outward to reach the ears. acetate arms flex about 2-10mm per side in comfort, which is where 4 and 20 come from: minus 4 is a front almost as wide as the head, minus 20 is a front well inside it with the arms doing the work. a frame wider than the temple track will not splay far enough to sit; much narrower than minus 20 and it grips.

eye size and the 18mm

eye size = (frame width − 18 − bridge) ÷ 2

frame width is two lenses, one bridge and two endpieces - the corners carrying the hinges. the 18mm is what the endpieces actually come to across this catalogue: the median of frame width minus (two lenses + bridge) over every frame whose front width we hold, and the commonest single value in it. it is measured from the frames rather than assumed from a material. thin metal runs tighter and heavy acetate wider, so treat the derived eye size as a guide to what to search for rather than a specification.

lens height and fitting height

the minimum of 34mm is a progressive rule in disguise. what a corridor needs is fitting height - pupil centre to the lowest point of the glazed lens - and the pupil sits roughly 55-60% up the lens on a typical fit:

fitting height ≈ 0.55 × lens height

so 34mm gives about 18mm of fitting height, which suits most modern progressives. designs with a shorter corridor - the channel the prescription changes through - will go lower.

the calculator does not offer an optical centre or a fitting height of its own. those depend on where the frame actually ends up sitting on your nose: bridge height, the profile of your nose, and pantoscopic tilt, which is how far the frame front leans back from vertical. a photograph taken from straight on cannot tell you any of them. that measurement belongs on the face with the real frame.

the capture protocol

a single frame is not a measurement. the reading is gated before it starts and taken across many frames:

head roll ≤ 8° · yaw asymmetry ≤ 0.12 · iris ≥ 11px · face centred in frame
then a 3 second settle, then 24 frames, and the median of each measurement

the middle value rather than the average, so one blink or wobble cannot drag the result. how much the 24 frames disagreed is shown alongside as a quality figure. a tight spread means you held still - it does not mean the answer is right. the bank card step gates tighter at 3° of roll, because a tilted card puts its 54mm height into a horizontal measurement at about 1% per degree.

face shape

eight shapes, decided by comparing four distances: brow width, cheekbone width, jaw width and face length. if the face is more than 1.5 times longer than it is wide, it is long. if the jaw is within 3% of the cheekbones, it is square or rectangular depending on that same ratio. a brow wider than the cheekbones with a narrow jaw is heart or triangle. and so on. it is a starting point for shape preference, not a clinical classification, and it is overridable - as is the colour suggestion, which samples forehead and cheek for skin, the iris interior for eye colour, and the scalp or brows for hair, then sorts warm against cool and light against deep.

the 36 styles

every frame in the catalogue is filed under one of 36 styles, which group into nine base shapes. the split is not even - the panto family alone is nearly half the catalogue, because that is what independent makers actually make.

boston 9 styles · 124 frames

the panto family. a soft keyhole-ish shape, slightly wider than tall, flat along the brow and rounded below. the safest shape on most faces, which is why it dominates the catalogue.boston · boston - keyhole · boston - square · panto · panto - bold · panto - round · panto - slim · panto - square · panto - wire

wellington 3 styles · 36 frames

the wayfarer outline - a trapezoid, wider at the top than the bottom, with a strong brow line. reads bolder than a panto at the same width.wayfarer · wayfarer - squared · wellington

round 6 styles · 31 frames

a true circle or close to it. draws attention to the eye and softens an angular face; wire versions almost disappear, acetate versions do the opposite.round · round - curved bridge · round - keyhole · round - large · round - saddle bridge · round - wire

browline 2 styles · 21 frames

heavy across the top, light or rimless below - the weight sits on your brow and lifts the face. the shape everyone recognises from the 1950s.browline · browline - slim

square 5 styles · 20 frames

corners kept, edges straightened. adds structure to a round or soft face and reads more formal than a panto of the same size.square - bold · square - geometric · square - oversized · square - rounded · square - soft

cateye 2 styles · 17 frames

the outer top corner sweeps up. lifts the eye and the cheekbone; the sharp version does it emphatically, the soft version barely at all.cat-eye · cat-eye - sharp

aviator 5 styles · 12 frames

a teardrop that is wider at the top and drops below the eye line. built for metal and for sun, and the only family here that regularly sits outside a rectangle.aviator - flat top · aviator - metal · aviator - navigator · aviator - round · aviator - teardrop

rectangle 2 styles · 3 frames

longer than it is deep, with square corners. the shallowest family, so worth checking against the lens height minimum if you wear progressives.rectangle · rectangle - slim

oval 2 styles · 2 frames

a circle stretched sideways. quiet and hard to get wrong, and rarer in current production than you would expect.oval · oval - wire

the style decides which drawing appears on the card and on the diagram. the base shape is what the face-shape guidance works with, so two styles in the same family are treated interchangeably when we suggest what suits you - the difference between them is a matter of taste, not of fit.

what changes when you try a frame on

more than you might expect, and less than would be ideal. three things happen:

the drawing is swapped for that style's own outline
the two halves move apart by (bridge − 20mm) × scale
the whole thing is pre-shrunk so the total still equals the real frame width

that third step is the one worth understanding. widening the bridge would otherwise make the frame wider overall, so the artwork is scaled down by exactly enough to absorb the gap. the lenses genuinely get narrower as the bridge gets wider - which is what happens on a real frame at a fixed outer width, and it is why a 49□21 and a 51□19 can both measure 143mm across.

so the outer width is true, the bridge gap is true, and the lens widths follow correctly from both.

what is not true to the frame is the lens depth. the drawing keeps its own outline's proportions vertically rather than scaling to the lens height we hold for that model. the height figure is still used - it is quoted on every card, it feeds the 34mm minimum, and it is on the printed sheet - but the picture does not reflect it. a shallow rectangle and a deep panto of the same width will draw with the same depth.

we would rather say so than let the drawing imply a precision it does not have. it is on the list to fix: the ratio is already in the catalogue for the frames whose makers publish it, so the drawing could be made height-accurate for those and derived for the rest.

what this is not

it estimates sizing from a photograph and a set of assumptions, several of which are named above as fitted to one person. it is not a dispensing measurement, it produces no optical centre, and it cannot see a nose in profile. confirm dimensions with the maker and have lenses fitted by a qualified optician.