When to use px vs rem vs em in css

CssComparisonBeginner · 4 min read

Px vs Rem vs Em in CSS: Key Differences and When to Use Each

Use px for fixed, precise sizes that don’t change with user settings. Use rem for scalable sizes based on the root font size, ideal for consistent responsive design. Use em for sizes relative to the current element’s font size, useful for nested scaling and component-based styling.

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Quick Comparison

Here is a quick table comparing px, rem, and em units in CSS based on key factors.

Factor	px	rem	em
Reference Point	Absolute pixels on screen	Root element's font size	Current element's font size
Scalability	No scaling, fixed size	Scales with root font size	Scales with parent element's font size
Use Case	Precise control, pixel-perfect UI	Consistent responsive sizing	Component-relative sizing
Accessibility	Ignores user font size settings	Respects user font size preferences	Depends on nesting, can compound
Nesting Effect	No effect	No effect	Sizes multiply with nesting
Common Usage	Borders, icons, fixed layouts	Typography, spacing, layout	Buttons, menus, nested components

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Key Differences

px is an absolute unit representing exact pixels on the screen. It does not change if the user adjusts browser font size or zoom, so it is best for elements needing exact control like borders or icons.

rem stands for "root em" and is relative to the root <html> element's font size, usually 16px by default. This means all rem values scale consistently if the user changes their browser font size, making it great for responsive and accessible typography and layout.

em is relative to the font size of the current element. This means if you nest elements with em sizes, the sizes multiply, which can be useful for component-based scaling but can also cause unexpected size growth if not managed carefully.

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Code Comparison

Here is an example using px to set font size and padding for a button.

css

button {
  font-size: 16px;
  padding: 10px 20px;
  border: 2px solid black;
}

Output

A button with fixed 16px font size and fixed 10px vertical and 20px horizontal padding, border 2px thick.

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Rem Equivalent

The same button styled using rem units, assuming the root font size is 16px.

css

button {
  font-size: 1rem; /* equals 16px if root font size is 16px */
  padding: 0.625rem 1.25rem; /* 10px and 20px converted to rem */
  border: 0.125rem solid black; /* 2px border */
}

Output

A button with font size and padding scaled relative to root font size, resizing if user changes browser font size.

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When to Use Which

Choose px when you need exact, unchanging sizes like borders, icons, or pixel-perfect layouts.

Choose rem when you want consistent, scalable sizing across your whole site that respects user preferences and improves accessibility.

Choose em when you want sizes to scale relative to the current element, useful for nested components or when you want local scaling effects.

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Key Takeaways

Use px for fixed, precise sizes that don’t scale with user settings.

Use rem for consistent, scalable sizes based on the root font size.

Use em for sizes relative to the current element’s font size, useful for nested scaling.

rem improves accessibility by respecting user font size preferences.

Avoid deep nesting with em to prevent unexpected size multiplication.