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.

CSS Basics

  1. Basics
  2. Tag Matching Tutorial
  3. Selector Syntax
  4. Units

Color

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  3. Opacity
  4. Linear Gradient
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Font

  1. font-family
  2. Standard Web Fonts
  3. font-size
  4. Meaning of Font Size
  5. Font Weight
  6. Chinese Font Names

Box Model

  1. Border
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  3. Margin vs Padding
  4. Box Sizing

Layout

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Box Decoration

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Misc

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  11. Declaring Character Set in CSS File
  12. Jargons Explained: Tag, Element, Node, Object, Attribute, Property, Method
  13. Display Table on Small Screen
  14. Fixed Aspect Ratio
  15. Add Icon to Links
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