charactercoverage

Character coverage is a measure used to describe how well a font, encoding, or input system can represent the characters used in a given language, text, or corpus. It reflects the extent to which the character repertoire of a font or encoding aligns with the characters that occur in practice.

There are two common viewpoints of coverage. Font coverage (or glyph coverage) assesses how many distinct characters in a target set are present as glyphs in a font. Corpus or language coverage looks at how many of the characters actually used in a text or set of texts can be rendered without fallback. In both cases, coverage is typically expressed as a percentage.

Calculation typically involves three elements: a target set of characters C (for example, the full set of characters used by a language or the characters in a specific document) and a available set G (the characters for which the font has actual glyphs, or the characters supported by the encoding). Coverage can be computed as |C ∩ G| / |C|. For text rendering, an alternate measure is the proportion of characters in a sample text that have corresponding glyphs in the font.

Applications of character coverage include guiding font selection for localization, evaluating font suitability for multilingual publishing, and assessing encoding adequacy in software development. It also informs font design decisions, such as whether to extend glyph repertoires or rely on fallback mechanisms. Limitations arise from dynamic fonts, platform-specific fallbacks, ligatures, diacritics, and contextual substitutions, which can affect practical renderability beyond static glyph counts.