CSS Units

One of the things you’ll use every day in CSS are units. They are used to set lengths, paddings, margins, align elements and so on.

Things like px, em, rem, or percentages.

They are everywhere. There are some relatively unknown ones, too.

Pixels

The most widely used measurement unit. A pixel does not actually correlate to a physical pixel on your screen, as that varies, a lot, by device (think high-DPI devices vs non-retina devices).

There is a convention that make this unit work consistently across devices.

Percentages

Another very useful measure, percentages let you specify values in percentages of that parent element’s corresponding property.

Example:

.parent {
  width: 400px;
}

.child {
  width: 50%; /* = 200px */
}

Real-world measurement units

We have those measurement units which are translated from the outside world. Mostly useless on screen, they can be useful for print stylesheets. They are:

cm a centimeter (maps to 37.8 pixels)
mm a millimeter (0.1cm)
q a quarter of a millimeter
in an inch (maps to 96 pixels)
pt a point (1 inch = 72 points)
pc a pica (1 pica = 12 points)

Relative units

em is the value assigned to that element’s font-size, therefore its exact value changes between elements. It does not change depending on the font used, just on the font size. In typography this measures the width of the m letter.
rem is similar to em, but instead of varying on the current element font size, it uses the root element (html) font size. You set that font size once, and rem will be a consistent measure across all the page.
ex is like em, but inserted of measuring the width of m, it measures the height of the x letter. It can change depending on the font used, and on the font size.
ch is like ex but instead of measuring the height of x it measures the width of 0 (zero).

Viewport units

vw the viewport width unit represents a percentage of the viewport width. 50vw means 50% of the viewport width.
vh the viewport height unit represents a percentage of the viewport height. 50vh means 50% of the viewport height.
vmin the viewport minimum unit represents the minimum between the height or width in terms of percentage. 30vmin is the 30% of the current width or height, depending which one is smaller
vmax the viewport maximum unit represents the maximum between the height or width in terms of percentage. 30vmax is the 30% of the current width or height, depending which one is bigger

Fraction units

fr are fraction units, and they are used in CSS Grid to divide space into fractions.