Cap Height Converter

Cap height is the distance from the baseline to the top of a flat-topped capital such as H, I or E, expressed here as a fraction of the em so it can be multiplied by any type size. It matters because the em square, which the font-size value actually sets, also contains invisible room for ascenders, descenders and built-in leading; only the cap height and x-height are visible on the page. Common text faces span a wide range - Courier New near 0.616, Times New Roman near 0.662, Lato near 0.694, Arial near 0.716, Verdana near 0.735 - so two faces set at an identical 16px can differ by more than 15 percent in apparent size. Two useful inversions follow from cap height equals size times ratio. To hit a specified physical cap height, divide the target by the ratio; to match a second typeface, divide the first face's cap height by the second face's ratio. Accuracy depends on where the ratio comes from: it should be read from the font's own OS/2 table, dividing sCapHeight by unitsPerEm, rather than estimated. Rounded capitals such as O and C overshoot the cap line slightly by design, so hand-measured values run a fraction high. And a cap height in pixels only becomes a physical dimension once display density is known - 12px is roughly 3.2mm at 96 DPI but half that on a 2x display, which is why print specifications given in millimetres need a stated resolution before they can be converted.

The named metrics of type - baseline, x-height, cap height, ascender, descender - grew out of the vocabulary of hand punchcutting and metal founding, where the visible height of capitals was a physical property of the punch rather than a number anyone published. Size itself was standardised long before the internal proportions were: Pierre Simon Fournier proposed a typographic point in 1737, Francois Ambroise Didot revised it in the 1780s, and the American Point System was adopted by US type founders in 1886, fixing the point at 0.013837 inch. None of those systems described how much of the body a capital actually occupied, which is why type specimens of the period had to be compared by eye. Digital font formats finally made cap height a machine-readable value. TrueType, developed by Apple and adopted by Microsoft around 1991, and OpenType, announced by Adobe and Microsoft in 1996, both carry an OS/2 metrics table; version 2 of that table added the sCapHeight and sxHeight fields, letting software read a face's capital height directly instead of measuring rendered pixels. That change enabled the modern practice of sizing text by cap height rather than by nominal font size, an approach popularised by web typography tools built on font-metric extraction, and reflected in CSS proposals for a cap unit and for font-size-adjust with a cap-height metric.