Weights and widths

"How many weights does this font have" sounds simple, but three vocabularies are in play at once: numbers inside the font, words in the family name, and style names in design software. They disagree constantly.

The numbers are a suggested scale#

OpenType stores weight in OS/2.usWeightClass, range 1–1000, conventionally multiples of 100:

The key word is "usual": convention, not standard. One font calls its regular 400, another 350. One "Book" is 400, another has Book at 350 and Regular at 400.

Names in the family are unreliable#

The name and the number may disagree:

• family says Medium while usWeightClass is something other than 500;
• both Demi and Semi Bold appear across foundries;
• 900 is Ultra to one foundry, Black to another, Heavy to a third.

So between "pick by name" and "pick by number", pick by how it renders. The number is an approximate label for software.

Width is a separate axis#

Width and weight are independent dimensions:

In CSS this is font-stretch, now more often written as a percentage: font-stretch: 75%.

Variable fonts turn the scale into a range#

A static family offers a few discrete weights; a variable font makes wght continuous (usually 100–900) and renders any value in between.

Practically: you no longer "pick one of five weights" but may ask for 437 — though nothing guarantees the drawing is good there, since the designer only checked a handful of positions.

Mapping to CSS#

A CSS font-weight number (1–1000) looks for a face declaring the same value, falls back to the nearest by a fixed rule, and finally synthesises if nothing fits.

Which means the same CSS can render differently across fonts, because "nearest face" differs. See font-weight and the bold alias.

How this site shows it#

The inspector reads usWeightClass directly. When the value was estimated rather than declared by the font, it carries a ≈ with an explanation — readable, but not a precise fact.

Width is shown when present and left blank otherwise; nothing is guessed.

Advice when choosing#

1. Judge by actual rendering (specimen or preview), not the name;
2. For weight contrast, make sure the weights really exist rather than being synthesised;
3. If Chinese and Latin come from different families, note that their 500 / 600 may differ visually;
4. For variable fonts, check whether a wght axis exists and its range (some only span 400–700).

Further reading#

• OpenType spec: usWeightClass and usWidthClass in OS/2
• MDN: font-weight, font-stretch
• This site: font-weight and the bold alias, maxp and variable axes