There are many different units that you can use to define sizes in CSS. This lesson will introduce the most important to you, and show you where to learn about the rest of them.
- You’ll learn the difference between relative and absolute units
- You’ll learn when it’s appropriate to use the different units.
Absolute units are those that are always the same in any context.
px is an absolute unit because the size of a pixel doesn’t change relative to anything else on the page. In fact,
px is the only absolute unit you should be using for web projects. The rest of them make more sense in a print setting because they are related to physical units such as
in (inch) and
Relative units are units that can change based on their context. There are several of them that you are likely to encounter and want to use.
em and rem
rem both refer to a font size, though they are often used to define other sizes in CSS. You’ll see both of them often so we’re going to explain both, but as a rule-of-thumb, prefer
1em is the
font-size of an element (or the element’s parent if you’re using it to set
font-size). So, for example, if an element’s
16px, then setting its width to
4em would make its width
16 * 4 == 64).
1rem is the
font-size of the root element (either
html). The math works the same with
rem as it did with
em, but without the added complexity of keeping track of the parent’s font size. Relying on
em could mean that a particular size could change if the context changes, which is very likely not the behavior you want.
Using a relative size like
rem to define font sizes across your website is recommended. Many browsers allow users to change the base font-size to increase readability. If at all possible, it is advisable to respect a user’s wishes regarding font size. You’ll learn more about this from the reading assignments.
vw relate to the size of the viewport. Specifically,
1vh is equal to
1% of the viewport height and
1vw is equal to
1% of the viewport width. These can be useful any time you want something to be sized relative to the viewport, examples including full-height heroes, full-screen app-like interfaces.
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This section contains questions for you to check your understanding of this lesson. If you’re having trouble answering the questions below on your own, review the material above to find the answer.