Understanding the Difference Between REM and PX in CSS

Published Mar 7, 2024

Written by: Abdulmumin Yaqeen

Last Edited: May 16, 2024


Two commonly used units are REM and PX. In this tutorial, we’ll explore REM and PX in depth, providing examples and scenarios to illustrate their usage and differences.

Understanding REM and PX:

REM (Root EM) and PX (Pixels) are both units of measurement in CSS, but they have distinct characteristics and applications.

1. Pixels (PX):

  • Absolute unit of measurement.
  • Fixed size unaffected by browser settings.
  • Ideal for defining precise dimensions.

2. REM (Root EM):

  • Relative unit based on the root element’s font size.
  • Adjusts based on the root font size, ensuring scalability.
  • Suitable for responsive design and accessibility.

Scenario 1: Responsive Typography

Imagine you’re designing a website where text sizes should scale proportionally with the viewport size:

html {
	font-size: 16px; /* Base font size */

body {
	font-size: 1rem; /* 1rem equals 16px */
	line-height: 1.5rem; /* 1.5 times the base font size */

h1 {
	font-size: 2rem; /* 2 times the base font size */

In this scenario, using REM ensures that text sizes adjust dynamically based on the root font size, providing a consistent and responsive typography experience.

When to Use REM:

For responsive design: REM units are ideal for creating designs that adapt smoothly to various viewport sizes and user preferences.

Accessibility: REM units accommodate users who adjust their browser’s default font size for better readability.

Consistency: REM units promote consistency in typography and layout across the website.

Scenario 2: Fixed-width Layout

Suppose you’re working on a design that requires fixed-width containers and elements:

.container {
	width: 960px; /* Fixed width container */

.button {
	padding: 10px 20px; /* Fixed padding */

.box {
	border: 1px solid #000; /* Fixed border */

Here, PX units are appropriate for defining fixed dimensions, ensuring elements maintain their sizes regardless of the viewport dimensions.

When to Use PX:

Fine-tuned control: PX units are suitable for elements that require precise dimensions, such as borders, shadows, and images.

Fixed layouts: PX units are preferred when designing fixed-width layouts that do not need to adjust to different screen sizes.

My Takes:

  1. Use REM for typography and layout dimensions that need to scale.
  2. Reserve PX for fixed dimensions and precise control.
  3. Maintain consistency within your project for a cohesive design.


Understanding when to use each unit allow you more flexibility when building for various devices and user preferences.

Tagged with: CSS rem px tips
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