Understanding key differences between absolute and relative units in CSS is crucial for building fluid layouts and responsive designs.

This article aims to demystify relative length units. In contrast to absolute length units (with px as the best known representative), relative length units specify a length relative to something else. This “something else” can be of various types, e.g., a parent element’s font size, the width of a parent container, or the height of the viewport.Both types of units have the term length in common, but what exactly is a length unit in this context? There are font-relative length units (e.g., em, rem), which are relative to characters or font-related properties of an element. Additionally, there are length units that are relative to the viewport (e.g., vw, vh).

Another common CSS data type in the context of relative units is the percentage (%). There are also CSS properties that accept integer values. The most common use case for such a unitless value is to use it with the line-height property.

Relative units are especially important from the perspective of fluid layouts and web accessibility to support users that rely on zooming. These fluid layouts are based on a proportional design, where lengths are defined in terms of percentages regarding a container.

Therefore, components based on relative units might change in size at runtime because they are (re)calculated with respect to a contextual container, e.g., by rotating a device or by decreasing the size of a browser window.

Font-related CSS units

First, we’ll look at how the most common relative font-related CSS units work: em and rem.

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