The main difference between em and rem in CSS is their reference point: em is relative to the font size of its own element or inherited from a parent, while rem is always relative to the root html element’s font size. 1. em scales based on the local font size, which can lead to compounding effects in nested elements. 2. rem provides consistent sizing by always referencing the root font size, avoiding unexpected scaling. 3. Use rem for layout spacing like margins, padding, and headings that should remain uniform. 4. Use em for internal component sizing where proportional scaling makes sense. 5. Both units support accessibility when not set with absolute values.
When styling web pages with CSS, choosing the right unit for sizing elements matters. Two commonly confused units are em
and rem
. While they may seem similar at first glance—both are relative units—they behave quite differently depending on context.

What’s the main difference?
The key difference lies in what each unit is based on:
-
em
is relative to the font size of its own element (or inherited from a parent if not explicitly set). -
rem
is always relative to the root element's font size (html
element), no matter where it's used.
This distinction affects how sizes scale throughout your page, especially when nesting elements.

How em
works
As mentioned, em
is relative to the element’s own font size or that of its closest ancestor. This makes it flexible but potentially tricky, especially in deeply nested structures.
For example:

html { font-size: 16px; } .parent { font-size: 20px; } .child { font-size: 1.5em; /* becomes 30px (1.5 × 20px) */ }
If you nest another element inside .child
and use em
, the multiplier keeps stacking. That can lead to unintended growth unless you're careful.
Use em
when you want something to scale relative to a component or module—like buttons or cards—where internal spacing or text should grow together.
Why rem
is more predictable
Since rem
always refers back to the root font size, it avoids compounding issues that come with em
.
Let’s say:
html { font-size: 16px; } .box { font-size: 1.5rem; /* always 24px (1.5 × 16px) */ }
Even if .box
is nested deep inside other elements with different font sizes, it still calculates based on the root value. This makes rem
ideal for layout spacing like margins, padding, or heading sizes that should remain consistent across the site.
A common practice is setting a base font size on the html
element (e.g., 10px) to make math easier:
html { font-size: 10px; } .title { font-size: 2.4rem; /* equals 24px */ }
When to use which
Here’s a quick guide:
-
Use
rem
for:- Consistent spacing and sizing across the whole site.
- Layout-related values (margins, padding, widths).
- Headings and UI components that shouldn’t scale unexpectedly.
-
Use
em
for:- Sizing within a specific component (like button text or icon sizes).
- Situations where scaling relative to local font size makes sense.
- Fine-tuning spacing inside a block that already has its own font size defined.
Also, keep in mind accessibility: using relative units helps users who adjust their browser’s default font size, as both em
and rem
respect those settings (as long as you don’t set html
font size in absolute units like px
).
A few practical notes
- You don't have to stick to one. Mix
em
andrem
where appropriate. - Avoid setting
html { font-size: ... }
inem
, because it can become unstable depending on user settings. - If you’re using a CSS preprocessor or design system, define a clear strategy early on to avoid confusion later.
基本上就這些。
The above is the detailed content of Difference between `em` and `rem` units in CSS?. For more information, please follow other related articles on the PHP Chinese website!

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