Meaning of Font Size

Let's say you use CSS to specify

font-size:16px

Ok, it's 16 pixels. But what does it mean exactly? is it height of M? or width of M? Neither!

What Does Font Size Mean Exactly?

The CSS font size of a font is the “em height”.

“em height” is the height of a imaginary design box around the font that includes space for ascender and descender plus a little padding.

“em height” is not the bounding box. (“bounding box” is not a concept in typography.)

The actual height of letter M, differs depending on the font used.

Some character, such as Ü , may extend beyond this design box.

Actual Letter Height

Different font with same CSS size spec may have very different size, because the ratio of CAPS height to “font size” is not standardized.

For example, compare:

Typically, serif font are smaller.