Glasses Temple Length: Four Printed Rungs, One Line on the Head, and a Six-Millimetre Overlap

The arm, the side of the head, and the twenty millimetres where they overlap

The word does two jobs in this family, and a single page can use both. The first job is a part of the frame: the arm that runs back from the hinge to the tip that sits behind your ear, the third of the three numbers stamped inside it, which this site's page about those numbers reads in order and which will not be repeated here. The second job is a part of your head. One how-to-measure page in this set asks for both jobs in the same list of steps: it sorts faces by the distance between the sides of the head — 109 to 132 mm (4.29 to 5.20 in) for a narrow face, 133 to 138 (5.24 to 5.43 in) for a medium one, 139 mm (5.47 in) and up for a wide one — and on the same page it gives the arm's range as 120 to 150 mm. Those two bands share twenty whole millimetre values, every integer from 120 to 139 (4.72 to 5.47 in). A number on a scrap of paper in a shop cannot say which of the two it is, and the sentence a wearer actually arrives with — that glasses arms too long keep sliding down — is not a measurement either, which is the reason this page starts by naming the part instead of the problem.

The doubling is not an accident of modern copy. A book about choosing spectacles whose own record gives the year 1921 spends a passage on the creases that start below the outer corner of the eye and run back toward the temples, and in that same passage it needs a second name for the arms — it calls them side pieces — because the word it would otherwise use is already busy describing your head. That is a vocabulary workaround, not a measurement, and it is older than the boxing system that put the first millimetre figures on a frame.

What the pages do print is settled and not in dispute. Six of them give the same adult arm band, 120 to 150 mm (4.72 to 5.91 in), in six different sentences. Two of those six also name the four common rungs as 135, 140, 145 and 150 mm, in 5 mm steps. A seventh page, one that stocks sunglasses model by model, puts thirteen different arm lengths across its tables, four of them shorter than the 120 mm floor those six treat as the bottom of adult stock. One page sells longer arms as their own category at 160 and 170 mm, outside the band, and files them as a special case rather than as the top of a ladder. The sizes are on the table. What is missing is the thing they are sizes of.

One page in this set puts its millimetres somewhere you can find with a finger. It is a consumer eye-health reference whose page top carries the date 18 January 2019 and whose most recent update line is from April 2026, and it says the arms should be long enough to bend downward at about 45 degrees at a point just past the top of the ears, with about 30 to 45 mm (1.18 to 1.77 in) of the arm running past that bend point and adjusted to hug the curve of the head behind the ear. Read as a purchasing rule rather than as a fitting bench, that sentence contains two quantities and one subtraction: the arm is the whole path, the path behind the bend is fixed at 30 to 45 mm, so the path in front of the bend — hinge to bend point, along the side of your head — is the stamp minus that figure. The table below runs it for every rung of the printed ladder. The first three columns need nothing but the published numbers. The last two are this page's own arithmetic on a two-segment model of the arm, and the note under the table says exactly what they assume and what they read if you move the angle.

Column three is flat because the published slack is flat: 45 minus 30 is 15 mm (0.59 in) of give, and fifteen millimetres of continuous run contains sixteen whole millimetre positions whatever the stamp. That single fact is what makes four rungs behave like one long one. Columns four and five are the ones to distrust: they assume an arm that is straight to the bend point and straight again after it, which no arm is, since a real temple curves to follow the head. On the 145 mm row the loss in column five reads 3.2 to 4.2 mm (0.13 to 0.17 in) if the bend is taken at 30 degrees instead of 45, and 12.4 to 16.5 mm (0.49 to 0.65 in) at 60 degrees. Across the seven rows the middle reading is 6.8 to 9.5 mm, which is between one and two of the 5 mm steps the ladder is built on. So column five is a warning about how you measure, not a size you can order.

Four rungs, thirty-one heads, and the one position all four fit

Each stamp covers sixteen whole millimetre positions of head, and each neighbour shifts the window by 5 mm (0.20 in), so consecutive windows overlap by eleven positions. Stack the four rungs the pages call common and what you get is not sixty-four options but thirty-one positions: the four together serve every run from 90 to 120 mm (3.54 to 4.72 in), and no head between those numbers gets four choices. Ask which of them is the right temple size on glasses and the honest arithmetic answer is that the answer is rarely unique and sometimes singular. Of those thirty-one positions, ten are served by exactly one of the four rungs, ten by two, ten by three, and exactly one position — a run of 105 mm (4.13 in) — is served by all four. That distribution is the thing a size chart cannot show, because a size chart lists frames and this is a count of heads.

At the edges the four rungs give out, and the printed band is wider than the common list. A run of 85 mm (3.35 in) is served by 120, 125 and 130 mm: all three sit inside the published adult band and not one of them appears in the four sizes two of the pages name as what you will find. A run of 125 mm (4.92 in) is served by nothing inside 120 to 150 mm at all — it needs 155 to 170 mm of arm, and the only rungs in this set that reach it are the 160 and 170 mm on a page that sells long arms as their own category, not as the top of a range. Read the whole seven-rung ladder and the arithmetic gives this: a band 30 mm (1.18 in) wide covers 45 mm (1.77 in) of head, sixty-four assignments collapsing to forty-six distinct positions from 75 to 120 mm (2.95 to 4.72 in). The ladder is narrower than the thing it is supposed to fit, and it is the subtraction that shows the gap, not the stamp.

The second table below works one column of the first one back. The rows are runs of head, the way you would arrive at them from a pair that fits you rather than from a frame in a shop window, and the cells name the stamped lengths that would be correct for that run. Where the common four run out the rest of the printed band is listed too, because a reader at the low end has more stock available than the four-size summary suggests — and a reader at the high end has none.

Read the third column down and then across: it rises to four exactly once. That single row is also the midpoint of the overlap between the two sizes a wearer asked about on 11 May 2023 was choosing between — 135 and 145 mm — which is why one of the answers under that question reported trying the longer arm and not being able to tell, while the arithmetic still says a head at 95 or at 110 mm cannot wear both. The 160 and 170 mm rungs come from a page updated on 21 April 2026; no page in this set prints a head run for them, because no page in this set prints the run at all.

The question this family actually asks, and the three answers it gets

The question rarely arrives as a request for an arm length in millimetres. The way it does get phrased — how long should glasses temples be — is directional, and these ten positions answer it in three ways, none of which reaches a head. The first is a copy operation: one retailer's measurements guide, whose machine-readable publication and modification stamps are both 23 September 2026, tells you to measure from the hinge screws to the tip of the arm; a fitting guide published on 25 September 2023 and revised sixteen days later gives the same instruction from the hinge to the tip and adds, as the whole of its unit guidance for that step, that you should write the number down. Both are sound instructions about an object in your hand. Neither produces a number that can be compared with anything except another number of the same kind. The second answer is a feeling: an optical glossary entry updated on 3 March 2026 whose subject is this one measurement says that the correct value keeps frames stable without slipping, pinching or creating pressure behind the ears, and prints no millimetre and no inch anywhere in the 485 words that page renders. The third answer measures your face and returns a figure that belongs to the front of the frame, which is a different quantity on a different line.

There is one account in the ten that asks the buyer's question in buyer's words, and it carries the date 11 May 2023. A wearer had found a pair offered in one size only, 135 mm, wore something longer, and wanted to know whether ten millimetres — one centimetre, that is how the question puts it — was a big difference. The most-endorsed answer in six comments offers no criterion: go to an eyeglass store and try the 135 mm ones. A second commenter reports wearing 140 mm, trying 145 mm and being unable to tell the difference. Both of those answers are honest, and the subtraction says precisely when each of them is right. The windows for 135 and 145 mm overlap across six whole millimetre positions, from a run of 100 to 105 mm (3.94 to 4.13 in): inside that range both sizes are correct and a wearer cannot feel the difference, which is the second commenter's experience reproduced exactly. At 95 mm (3.74 in) only the shorter is correct; at 110 mm (4.33 in) only the longer is, and the longer one is not offered. A shop visit settles it either way, but it settles it by trial, and the trial is six millimetres wide.

The reason the symptom vocabulary survives is that the geometry is genuinely small where the fit is genuinely good. The same 18 January 2019 reference that prints the bend rule hands the last step away: in most cases, it says, a skilled optician can make the proper adjustments. That is a true sentence and it is also the reason no retail page has ever needed to convert anything, because the person doing the converting is standing behind a counter with a heater. The pages that try to close the gap anyway give direction without magnitude — frames whose arms are much shorter than needed press more at the ears, much longer and they feel looser — which is the published state of the art, and it is a statement about how the tail behind the bend behaves rather than about the run in front of it.

One more line deserves quoting because it is the closest any of these pages comes to a claim about the number itself. The retailer's guide stamped 23 September 2026 says of the arm that this measurement does not vary as much as the others. Its own band is 30 mm wide, and by the subtraction above those 30 mm of stamp buy 45 mm of head, which means the third number is the least variable of the three in the way that matters least: there are fewer rungs per millimetre of the thing it has to fit.

Getting the run of your own head, which is the number nobody prints

Everything on this page turns on one quantity that no page in this set publishes: the distance from the hinge of a frame that sits correctly on you to the point where its arm starts to bend down, measured along the side of your head. Two routes reach it, and neither is a tape across the face. The first works off a pair that already fits: find the bend — on a correctly fitted arm it is the point where the metal or plastic changes direction, which is the published position of just past the top of the ear — mark it with a pencil line on a strip of masking tape stuck to the arm, unfold the arm flat, and read hinge screw to that mark. That number goes into column two of the first table, and the row whose window contains it is your size. The second works when no pair fits you: hold the frame you are considering against your face, find where the arm crosses the top of your ear, and take the distance along the side of the head from the hinge position to that point with the tape running down the skull rather than across the face.

Both routes have one failure mode, and it is not the one you would guess. A tape that follows the curve of the skull reads long against the straight line, because the arc exceeds the chord; on runs of 90, 100 and 105 mm with the tape standing 5 mm off the straight line at its midpoint, the excess is 0.74, 0.67 and 0.63 mm — under a millimetre, and less than a seventh of the 5 mm step between rungs. Press the tape harder and let it stand 10 mm off the line over a 100 mm run and the excess reaches 2.65 mm (0.10 in), still inside one step. So bowing is a rounding error here, and the error that actually costs you a size is picking the wrong line: reading across the face instead of down the side returns a figure from the band that overlaps the arm's own band by twenty whole millimetres, which looks exactly like a valid answer. A run of 105 mm and a face reading of 105 mm are different measurements of different objects and only one of them can be subtracted.

What the run cannot do needs saying before it is used. It says nothing about the width of the front, which is constrained by your face and by the two numbers in front of this one, and that arithmetic — including what a front has to add up to and what an allowance for the corners of the frame costs — is worked out on this site's page about measuring a frame front and is not repeated here. It has nothing to say about where your pupils land inside a lens, which is the subject of the page about the numbers stamped on the arm. The run is a third line on the body, parallel to neither of those, and the only reason it decides anything is that the published rule for the arm is the only rule in this family that has an anatomical landmark in it.

An 1850 medical treatise on spectacles is the oldest position in the table at the end of this page, and it is worth reading against the retail advice two centuries later because it makes the same argument without a single number. Its case is that the framework should be light and correctly dimensioned, that a frame too large and broad loses its steadiness and tires the eyes through the movement of the image, and that one too narrow and heavy presses the temples and produces pain in the parts near the eye. Too loose and too tight, both stated, both sides of a fit described as sensations with a location, and the arm never separated from the front. The vocabulary has been stable for a very long time; the millimetres are the part that arrived late, and the conversion between them is the part that has still not arrived.

Why a tape on a worn arm reads short, and where the arm's zero sits

The instruction that appears on two of these ten pages — measure from the hinge to the tip — describes a path, not a straight line, and one of those two pages is explicit that the bend counts: its own definition sentence calls the figure “the length of the temple from the hinge to the temple tip, including the bended part that sits on your ear”. A third page, one updated on 21 April 2026, puts the same point as a rule for the tape: the measurement runs end to end including the bend, and it calls that one of the key numbers printed on the frame. Those are correct descriptions of the stamp, and they are the reason nobody should follow the instruction on a pair of glasses sitting on a face. A bent arm measured with a straight tape from hinge to tip returns the chord, and the first table's last two columns show how far short that is: 6.8 to 9.5 mm (0.27 to 0.38 in) across the ladder, which is a size and a fifth to nearly two sizes depending on where in the band you are. The failure is directional, not random. The chord is always shorter than the path, so the error always pushes a shopper toward a smaller number — toward the size that will not reach past the ear by the 30 to 45 mm the rule asks for.

Work an example, because the arithmetic is small enough to hold in your head. An arm stamped 145 mm, straight to the bend point and then running down at the published 45 degrees, puts its tip between 135.6 and 137.9 mm (5.34 and 5.43 in) from the hinge in a straight line, depending on whether 30 or 45 mm of it lies behind the bend. A shopper with a tape on a worn pair reads something in the 136-to-138 range, rounds it toward the rungs on offer, and buys a 135 mm arm for a head that needs 145. The stamp did not change and neither did the head; the measuring method converted a correct frame into a wrong order. Unfolding the arm flat costs nothing and removes the entire error, which is why the copy-the-number-down instruction only works on a frame lying on a table.

The zero point of that measurement is another quantity none of these pages names, and it matters because it is where the arm stops being independent of the front. The hinge is not the outer edge of the frame face; between them sits the corner piece the hinge is mounted into, and the width of that corner — an endpiece width, on the rare specification sheets that bother with corners at all — is what decides how far outside the lens the arm's start actually is. Across these ten pages the word appears nowhere, including on the six-table size chart that lists every other dimension model by model, and including on the two pages whose measuring instruction begins at exactly that point. What an allowance for it costs a total front width is computed on this site's page about the stamped numbers and its page about measuring a frame; neither figure is reproduced here, and the arm arithmetic on this page does not need it, because a hinge is a hinge at either end of a frame.

One caution about the two-segment model in columns four and five. It treats the arm as two straight runs meeting at a point, which is what makes the deficit computable and is not what an arm looks like: most temple tips curve continuously, and a frame with a swept hinge puts the bend somewhere other than where you will mark it. The model's numbers survive as an order of magnitude — between one and two size steps of error, which is the whole argument for unfolding the arm — and should not be used to predict what your tape will say on your frame. The arc figures in the section above are firmer, because they bound an error rather than describing a shape.

The adjustable number, which no shop can adjust for you before you buy

Of the three numbers in a stamp, the arm is the one a frame can be changed to match, and this site has already published what that costs: the page about the numbers on the arm records that it is the one of the three you can have adjusted on a metal front without touching the optics, and the page about glasses for a heart-shaped face records that bending an acetate front at a counter does not move the problem, because the curve behind the ear is set with heat. Neither claim is argued again here. What follows from them is specific to the run of head, and it is the reason the thirty-to-forty-five millimetre slack is worth more on some frames than on others.

A metal arm that is bent at the counter does not change its stamped length — path length is what it is — but bending moves the point where the path turns downward, which trades arm between the run in front of the bend and the tail behind it. Every millimetre the bend point travels backward is a millimetre taken off the run and added to the tail, and a correct adjustment is the one that puts your run in column two by moving the bend rather than by changing the frame. That is what the 18 January 2019 rule means when it says the portion behind the bend should be adjusted to conform to the curve of the head, and it is also why the same sentence hands the job to a person with tools. On a rigid acetate front the bend point is fixed, the trade is unavailable, and your run has to be inside the stamped window at the moment of purchase — which is the difference between a size being a starting point and a size being a verdict.

There is a limit to what any of this reaches, and a buyer should hold it next to the optimism of a size chart. Adjustment fixes where the arm turns down; it does not fix a frame whose front is 20 mm wider than your face, and that is the job of a different number on a different line. It does not make a 120 mm arm fit a head whose run is 125 mm, because no amount of bending lengthens the path. And nothing here says a frame in the correct rung will feel right: pressure at the bridge, the weight of the front and the angle of the pad are not on any column of this page. The run decides whether the arm can reach the ear with enough tail to hold it there. Everything after that is a fitter's judgement, and on a plastic frame it is a purchase decision made before the fitter is ever involved.

What a shelf prints, and what it cannot print

The most numerically generous page in this set is a sunglasses size chart with six tables and a row per model, and it prints an arm length for every frame it sells: thirteen distinct values between 100 and 150 mm (3.94 to 5.91 in), including 124 and 128 mm, which appear on no ladder any other page publishes, and four values below 120 mm — 100, 110, 115 and 118 (3.94, 4.33, 4.53 and 4.65 in) — which sit under the floor that the six pages printing an adult band treat as its bottom. Its own guidance for choosing among them is a single wide sentence: it is probably between 100 and 150 mm. Read the first table's column two across all thirteen values and the shelf covers runs of head from 55 to 120 mm (2.17 to 4.72 in), sixty-six whole positions; the adult band alone covers forty-six of those. The stock is finer than the summary and the summary is the only advice attached to it.

A shelf can publish what it sells and still not tell you what fits, and the gap has an edge that can be named, because it is the gap this page is about. Every arm length on a specification is a path length on a frame. To use it you need a path length on your head, and the only published bridge between the two is the bend rule with its 30 to 45 mm tail, which converts a head measurement into a band of stamps. Without that conversion a shopper with a size chart is comparing a frame's number against another frame's number — the arm on the shelf against the arm on the pair at home — which is sound only if the pair at home fits, and useless if the pair at home is the reason you are shopping.

The other way a shelf fails is by answering the right question with the wrong line. A retailer's frame guide prints a ladder of 120, 130, 140 and 150 mm (4.72, 5.12, 5.51 and 5.91 in) and then decides between small and large by the width of your face: under 130 mm (5.12 in) at the widest point takes the small pair, over 140 mm (5.51 in) takes the large. That is a defensible rule about the front and an undefined one about the arm — nothing in the sentence connects the face figure to the third number, and the two are related only through everything the two other pages on this site work out about fronts. A second page in this set does the same thing more briefly, attributing its own band to another publisher rather than to a measurement, and a third gives direction without magnitude: press more at the ears if short, feel looser if long. Three pages, one ladder each, and no line from a head to a rung on any of them.

Three steps, in the order they can actually be done

Take the run first, because every later step is arithmetic. Use the pair you wear now if it does not slide and does not press: mark the bend with tape, flatten the arm, read hinge screw to mark, and treat that figure as your run in millimetres — not the stamp on the leg of that frame, which is a path length and not the same quantity. If you have no frame that fits, hold a candidate against your face and read the distance from the hinge position down the side of your head to the top of your ear. Either way, take the measurement twice with the tape flat rather than riding a curve, and write down the millimetres, not the size you expect.

Put that number in column two of the first table and read the row that contains it. If your run falls on one of the ten positions served by a single rung, buy that rung. If it falls in an overlap, you are not choosing between a correct size and a wrong one; you are choosing how much tail sits behind your ear, and the tiebreaker is whether anything can adjust it later, which is the metal-and-heat difference this site has already drawn and which no online shop can settle for you. On a rigid front, where nothing moves after purchase, the shorter of two correct rungs is the safer buy: a tail that is too long can be bent down more aggressively than an arm that never reaches behind the ear.

Then hold the result against what this page cannot tell you. Your run says nothing about whether the front is the right width, which is constrained by your face and is the subject of the page about measuring a frame; and it settles nothing about whether a laboratory can centre a prescription in the lens that front carries, which belongs to the page about the stamped numbers and the page that starts from your pupils and solves for the width. Two of those three pages will tell you that a size you can wear is not a size you can order lenses in. A correct arm on a front that cannot carry your prescription is still a return, and the arm being right is the smallest part of the purchase that a ruler can settle.

Twelve published positions, eight of them dated

The positions above are all taken from the pages themselves, and this last table orders them by the date each carries rather than by whether they agree. Two of the twelve are books and carry a year only; one of those is a medical treatise on spectacles whose own record gives 1850, which makes the span between the oldest and the newest dated position here between 64,184 and 64,548 days, depending on which day of that year the book left the press — about a century and three quarters, and longer than the boxing system has existed by most of it. Narrow the span to the sources that carry a day as well as a year and six positions remain, from the eye-health reference published on 18 January 2019 to the retailer's measurements guide stamped 23 September 2026: 2,805 days, which is seven years, eight months and five days.

The last column is the one to read, because it says what each source's own instruction leaves you holding, and across a century and three quarters it barely moves. The 1850 treatise leaves you with two sensations and their locations. The 1921 book leaves you needing a second word for the arm so it can talk about the side of a head. The 2019 reference is the only position that leaves you with a number you can subtract, and it presents that number as work for somebody else. The 2023 question leaves you with a shop visit. The 2026 glossary leaves you with the word stable, and no length at all. Eight of the twelve print a millimetre figure, and one of those eight also prints a way to reach a measurement on a head; nobody in the whole table does the arithmetic between them, which is the only claim this page is making.