Meaning of Font Size

By Xah Lee. Date: . Last updated: .

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!

font size em height
Font size design box.

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.)

typography line terms
typography line terms

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:

Bye Bye

Typically, serif font are smaller.

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