The printed 14 to 24 mm band drawn as a ladder: what each stamped bridge does to the lens centres, to decentration and to the front's width at a fixed 52 mm lens, the two guessing rules crossed into four combinations, and the five whole millimetres no mirror rule reaches. Print it or keep it on your phone.
| What it measures | The gap between the two lenses, read at the openings the lenses sit in. A boxing-system reference dated 10 June 2026 defines the figure as the shortest distance between the nasal edges, and that is the number that lands in the middle of a temple arm — the aperture figure, not the rim figure. |
| What it is not | The bridge opening, and not a measurement of your nose. Three published instructions place the ruler on three different boundaries — across the inside faces of the two rims, which includes the rim walls, and between the openings the lenses sit in, which excludes them. What separates the two readings is a pair of rim walls, a quantity no page in this family prints. |
| What else decides fit | Two quantities no frame prints: the thickness of the frame at the ridge and the curvature the front carries. A wearer's account names them beside the stamped figure, which is why a 17 mm bridge can fit where a 20 mm one will not, and why one buyer reports frames from 17 to 24 mm that all fit. |
| Bridge, as stamped | Distance between lens centres | Decentration per eye at 63 mm | Hinge to hinge | Front, at the 20 mm allowance |
|---|---|---|---|---|
| 14 mm (0.55 in) | 66 mm (2.60 in) | 1.5 mm (0.06 in) inward | 118 mm (4.65 in) | 138 mm (5.43 in) |
| 16 mm (0.63 in) | 68 mm (2.68 in) | 2.5 mm (0.10 in) inward | 120 mm (4.72 in) | 140 mm (5.51 in) |
| 18 mm (0.71 in) | 70 mm (2.76 in) | 3.5 mm (0.14 in) inward | 122 mm (4.80 in) | 142 mm (5.59 in) |
| 20 mm (0.79 in) | 72 mm (2.83 in) | 4.5 mm (0.18 in) inward | 124 mm (4.88 in) | 144 mm (5.67 in) |
| 22 mm (0.87 in) | 74 mm (2.91 in) | 5.5 mm (0.22 in) inward | 126 mm (4.96 in) | 146 mm (5.75 in) |
| 24 mm (0.94 in) | 76 mm (2.99 in) | 6.5 mm (0.26 in) inward | 128 mm (5.04 in) | 148 mm (5.83 in) |
The allowance is this site's own number and only the last column contains it: at 12 mm for the pair the same six fronts read 130 to 140 mm (5.12 to 5.51 in) instead of 138 to 148, and not one other figure in the table moves. Column three steps 1 mm (0.04 in) per eye for every 2 mm of bridge, because halving the excess of a sum is what the formula does. Against the two ceilings in print, column three clears the 5 mm (0.20 in) figure in four rows, the 4 mm (0.16 in) end in three, and the 3 mm (0.12 in) end in two — the 14 and 16 mm bridges. The 14 to 24 mm band is 10 mm wide while the published 40 to 60 mm lens band is 20 mm wide.
| What the bridge reads against the pupils | What the nose is rated | The window both rules allow together |
|---|---|---|
| At or below the pupils | Narrow | 16 to 18 mm (0.63 to 0.71 in) — three whole millimetres, and the only wide agreement available. |
| At or below the pupils | Broad | 18 mm (0.71 in) — the two windows touch at exactly one whole millimetre. |
| Above the pupils, or wider than average | Narrow | Nothing — 19 to 21 mm and 14 to 18 mm do not overlap by a single millimetre. |
| Above the pupils, or wider than average | Broad | 19 to 21 mm (0.75 to 0.83 in) — and this is the window that lands highest on the ladder above. |
The empty cell is the finding, and it is internal to one page: the source that supplies the nose-width rule also supplies the pupil-height rule, in the same list of steps, so a bridge that reads above the pupils with a narrow nose is left holding two instructions that cannot both be followed. The pupil rule, applied to the printed band, can only ever return six whole millimetres — 16, 17, 18, 19, 20, 21 — so five of the eleven integers the same pages publish, 14, 15, 22, 23 and 24 mm, are unreachable by the most-reprinted fitting instruction in this family. The nose-width rule can reach the 14 and 24 ends, and it overlaps its own two bands at 18 mm without saying what to do there. Run through the ladder above at a 52 mm lens, the low window costs 2.5 to 3.5 mm (0.10 to 0.14 in) of decentration per eye and the high window 4.0 to 5.0 mm (0.16 to 0.20 in).
Multiply the two published bands instead of picking from them: a lens from 40 to 60 mm and a bridge from 14 to 24 mm produce lens-centre distances from 54 to 84 mm (2.13 to 3.31 in). That is a space 30 mm wide, and every combination in it is inside the adult ranges as printed.
| Ceiling | Sums admitted | Of the printed 30 mm | Left outside |
|---|---|---|---|
| 5 mm | 53 to 73 | 19 mm | 11 mm, all of it at the wide end |
| 4 mm | — | 16 mm | 14 mm |
| 3 mm | — | 12 mm | 18 mm — three fifths of the space |
Where the 3 mm reading's 18 mm of exclusion sits is not symmetrical: 3 mm of it is at the narrow end and 15 mm at the wide end. The narrowest combination in print — a 40 mm lens and a 14 mm bridge, 54 mm between centres — is not safe just because it is small: at 63 mm it puts each lens 4.5 mm (0.18 in) outward, which clears the 5 mm reading and fails both ends of the tighter band.