fontMetrics

Font metrics are measurements that describe the vertical dimensions of a font and how glyphs are laid out on a baseline. They are used by typesetting and text rendering systems to position glyphs and determine line spacing.

Key metrics include ascent (the height above the baseline of most ascenders), descent (the depth below the baseline of most descenders), and line gap or leading (additional space between lines). Cap height measures the height of capital letters, while x-height measures the height of lowercase letters like x. The em square is the design square that defines the coordinate space for font metrics, with metrics expressed in design units per em (unitsPerEm). During rendering, these design units are scaled to the chosen font size.

In font files such as OpenType or TrueType, metrics are stored in tables, notably the horizontal metrics tables and the OS/2 and head tables. These provide values such as hhea ascender/descender/lineGap, sTypoAscender/Descender/LineGap, and win metrics, which help renderers compute line height and vertical alignment.

Metrics influence the computation of line height and the vertical placement of glyphs within a line, as well as cross-font alignment and diacritics. Differences in metrics across fonts can affect readability and layout, so software typically exposes APIs to query a font’s metrics and to adjust line height accordingly.