font-relative length
CSS defines ex relative to the x-height of the element’s font; it varies by font even at the same point size.
Converter
font-relative length
em
exact
- Record
- M548
- Definition quality
- Standardized
- Conversion mode
- contextual
- Calculator
- Context required
Definition
In CSS, 1em equals the computed font size of the element; it therefore has no universal millimetre value without a font-size context.
History & use
The term descends from metal-type practice, but modern specifications define it by font size rather than literally by the width of the letter M.
Modern use
Modern use is context-dependent and varies by region or measurement system.
Sources & evidence
Source tier — · confidence high