HTML - How many points are in an EM?

Summary
1em
equals
12pt
equals
16px

Why?

1) The em is the font-size of the current font.

It is a relative measure, so:
1 em equals 1x the height of the current font.
2 em equals 2x the height of the current font.


If you haven't changed the current font, then 1 em will be the size of the browser's default font, which is 12 pt.

2) The point is a unit of measure from printed type, and is equal to 172 of an inch.

Which means the height of a 12 pt font is: 12 * 172 inch = 16 inch.


3) To determine the font height on screen, versus the printed page, we need to convert 16 inch to pixels.

The CSS specification defines a pixel to be 196 inch.

And 16 inch * 96 pixels per inch = 16 px.


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© Richard McGrath