Width: how much room the letters take

How font.io measures font width — the average advance of the lowercase letters, from condensed to extended.

Oswald: 0.40 — condensed. Archivo Black: 0.61 — extended. Inter: 0.54 — regular. Width is the average amount of horizontal room the lowercase letters take up, as a share of the font’s size. Three buckets: condensed, regular, extended.

Width: mean lowercase advance

Oswald a–z — each dimension line spans one letter’s advance width; the dashed line is the mean

Mean advance ÷ em = the width ratio. Buckets: condensed <0.46, regular ≤0.60, else extended. Advances here come from the font’s hmtx table and agree with measure.py’s PIL advances.

The measurement: take all 26 lowercase letters, a through z, measure how much horizontal space each one advances, average them, and divide by the font’s size. Using the whole alphabet instead of one letter keeps a single wide or narrow glyph from deciding the verdict.

A real example: Oswald’s lowercase averages 0.3999 — condensed, the narrow grotesque look it’s known for. Inter averages 0.5363 — regular. Archivo Black averages 0.6133 — extended.

No caveats needed here — the failure mode is simply missing data. If a font has no lowercase letters, or it’s a CJK font where every character is full-width by design, there’s no width number to report, so we leave it blank instead of inventing one.