Rem vs. Em: When Should You Use Each CSS Unit for Font Sizing?

Understanding the Difference Between Rem and Em in CSS

In web development, the choice between rem and em units for font sizing can impact the layout and responsiveness of your page. Let's explore how these units differ and their relative uses.

Rem vs. Em Units

Both rem and em units are relative units used to define font size or other properties in CSS. However, they differ in their relationship to their parent elements:

Em Units:

• Relative to the font size of the parent element
• A value of 1 em equals the current font size
• Changes in the parent's font size also affect child elements with em values

Rem Units:

• Relative to the base font size defined in the root (or specified by or elements)
• A value of 1 rem equals the root font size
• Changes in parent font sizes do not affect child elements with rem values

In the Provided Example:

In the code snippet you provided, the

element has a font-size of 1.4rem. This means that its font size is 1.4 times the base font size.

However, the

element within the

has a font-size of 1.4rem as well. Since

is a child element of

, if em units were used, the font size of

would be 1.4 times the font size of

, or 1.4 * 1.4 = 1.96 times the base font size.

Practical Considerations:

• Rem units are recommended for consistent sizing across multiple levels of nested elements. They prevent cascading font size changes when intermediate containers alter their font size.
• Em units are useful when you want to inherit the font size of a parent element but scale it slightly.
• Both rem and em units allow for easier scaling of fonts when the browser window is resized, ensuring consistency in the layout.

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