The three figures stamped inside a temple arm: which slot is which, the one sum the first two settle, the window every stamped size opens, and the two fit numbers the stamp does not carry. Print it or keep it on your phone.
Read the inside of a temple arm and you have three figures: a lens width, a bridge width, a temple length. They are stamped in that order, written either with dashes or with a small square between the first two, so 54-20-140 and 54□20-140 name the same front.
| Slot | In the example | What it is | Published adult range |
|---|---|---|---|
| Lens width | 52 mm | The width of one lens. A lens 52 mm across has an effective diameter of at least 52 mm. | 40 to 60 mm (1.57 to 2.36 in) |
| Bridge width | 18 mm | The gap between the two lenses. It is the second half of the sum in section 2 and the only other figure an optical calculation touches. | 14 to 24 mm (0.55 to 0.94 in) |
| Temple length | 140 mm | Where the arm bends behind your ear. It enters the decentration sum nowhere, and it enters the blank sum nowhere. | 120 to 150 mm (4.72 to 5.91 in) |
The three adult ranges were published on 18 January 2019 and were still published with the same figures on that page's April 2026 update. Four rows of the table in section 3 carry 135 and 145 mm temples, and no column of it moves, because the third figure is not an operand.
Add the first two figures and you have the frame's own pupillary distance. Subtract your own distance from that sum and halve it, and you have how far the laboratory has to move each lens on that front — the only fit decision that comes off the arm before anybody measures your face.
| Step | The arithmetic | In the example |
|---|---|---|
| Frame PD | lens width + bridge width | 52 + 18 = 70 mm (2.76 in) |
| The gap to your eyes | frame PD − your pupillary distance | 70 − 63 = 7 mm |
| Decentration per eye | the gap ÷ 2 | 3.5 mm (0.14 in) inward |
The published example runs the same sum at a 64 mm (2.52 in) pupillary distance: 52 + 18 = 70 mm of frame PD, and each lens is moved inward by 3 mm (0.12 in). Every millimetre of lens width moves the decentration by half a millimetre, and every millimetre of your own distance moves it by half a millimetre the other way. A frame PD below your pupillary distance gives a negative answer: the centres have to travel outward instead.
A limit of L millimetres per eye opens a window from frame PD − 2L to frame PD + 2L. The conversion is arithmetic; the three limits are published figures, and they do not agree with each other.
| Limit, per eye | In inches | Where it is printed |
|---|---|---|
| 5 mm | 0.20 in | A lens-laboratory technical guide, 9 March 2026: keep decentration under 5 mm per eye where the frame allows. |
| 4 mm | 0.16 in | A frame-styling guide, 3 April 2026 — the looser end of a “3–4 mm per eye when possible” band. |
| 3 mm | 0.12 in | The tighter end of the same band, published twenty-five days after the ceiling above it. |
| Stamped | Frame PD | Inside 5 mm | Inside 4 mm | Inside 3 mm | At 63 mm PD |
|---|---|---|---|---|---|
| 46-15-135 | 61 mm (2.40 in) | 51 – 71 mm | 53 – 69 mm | 55 – 67 mm | 1.0 mm (0.04 in) outward |
| 48-17-140 | 65 mm (2.56 in) | 55 – 75 mm | 57 – 73 mm | 59 – 71 mm | 1.0 mm (0.04 in) inward |
| 50-16-140 | 66 mm (2.60 in) | 56 – 76 mm | 58 – 74 mm | 60 – 72 mm | 1.5 mm (0.06 in) inward |
| 51-19-140 | 70 mm (2.76 in) | 60 – 80 mm | 62 – 78 mm | 64 – 76 mm | 3.5 mm (0.14 in) inward |
| 52-16-145 | 68 mm (2.68 in) | 58 – 78 mm | 60 – 76 mm | 62 – 74 mm | 2.5 mm (0.10 in) inward |
| 52-18-140 | 70 mm (2.76 in) | 60 – 80 mm | 62 – 78 mm | 64 – 76 mm | 3.5 mm (0.14 in) inward |
| 52-18-145 | 70 mm (2.76 in) | 60 – 80 mm | 62 – 78 mm | 64 – 76 mm | 3.5 mm (0.14 in) inward |
| 54-18-140 | 72 mm (2.83 in) | 62 – 82 mm | 64 – 80 mm | 66 – 78 mm | 4.5 mm (0.18 in) inward |
| 54-20-140 | 74 mm (2.91 in) | 64 – 84 mm | 66 – 82 mm | 68 – 80 mm | 5.5 mm (0.22 in) inward |
| 56-17-145 | 73 mm (2.87 in) | 63 – 83 mm | 65 – 81 mm | 67 – 79 mm | 5.0 mm (0.20 in) inward |
Inches are the millimetre figures divided by 25.4. The adult pupillary-distance band a hospital health article published on 17 April 2026 runs from 50 to 70 mm, so a window that extends past 70 mm is not a size you can wear at that end. The two ceilings sit 1 to 2 mm apart per eye, which is 4 mm (0.16 in) of pupillary distance at the edges of every row. One front is inside the looser ceiling for a 20 mm (0.79 in) band of faces and inside the tighter one for 12 mm (0.47 in). Rows four and six are the instructive pair: 51-19-140 and 52-18-140 have the same frame PD, so their windows are identical.
Two of the four things a frame has to satisfy are not on the arm at all. Depth — the vertical height of the lens opening — is almost never printed, and total width is never printed, because the two millimetres of metal or acetate at each end of the front vary by material and by manufacturer.
| Missing | What it costs |
|---|---|
| Total width | On the sizes in the table above it runs 58 to 71 mm (2.28 to 2.80 in) larger than the frame PD — a lens width plus the two end-piece allowances. A 52□18 front that measures 138 mm (5.43 in) end piece to end piece is its 70 mm frame PD plus the lens width plus a 16 mm allowance. Subtracting a pupillary distance from that instead of from the frame PD gives an answer in tens of millimetres where the true figure is a few. |
| Depth | The required blank is effective diameter + (frame PD − your PD) + 2 mm, and depth is the only moving part in it once the front and your distance are fixed. The same 52□18 front on a 63 mm (2.48 in) face needs a block of at least 61 mm (2.40 in) before any decision about depth or shape is taken. |