Font unit

A font unit measures distances in a computer font's design coordinates. If one em contains $U$ font units, a coordinate difference $q$ represents $q/U$ of an em. OpenType stores $U$ in the unitsPerEm field of its font-header table; its permitted range is 16–16384.^[ref]

The design coordinate is independent of a particular screen. Before grid fitting, an unrotated uniform scale with $p$ pixels per em sends a horizontal length $q$ toFor example, $U=2048$ and $p=12$ send a length of 512 font units to 3 pixels. Doubling the requested font size doubles that ideal geometric length without changing the stored outline.

A font unit is consequently not itself a pixel. Grid fitting can further change a scaled outline or its glyph advance to suit the pixel grid. Nor does the height of every visible glyph equal one em: the em fixes a scale, while individual glyphs have their own bounding boxes.

Pixels per em specifies the raster scale of one em. If $U$ font units make an em and the requested scale is $p$ pixels per em, a design length $q$ becomes $pq/U$ pixels before grid fitting. The value measures the nominal typographic scale rather than the ink height of an individual glyph.^[ref]

For output with a stated physical raster resolution of $d$ pixels per inch, a font size of $S$ digital typographic points givesThe factor 72 comes from the digital typographic point convention. For example, $S=12$ and $d=144$ give 24 pixels per em; a hypothetical glyph advance of 0.6 em then scales to 14.4 pixels. Doubling $d$ doubles the pixel count while preserving the requested physical length.

Horizontal and vertical pixel density can differ, in which case the two directions have distinct pixels per em. An application can also impose its own screen-to-physical-unit mapping. One must state which scale is being used rather than infer the physical size from a pixel count alone. Rasterization and grid fitting then determine how the scaled geometry becomes a visible image.