REM vs PX: The Complete Developer's Guide

January 27, 2025 • 8 min read • By Peter Csipkay

Choosing between REM and PX units is one of the most fundamental decisions in CSS, yet it's often misunderstood. This comprehensive guide will help you understand when and why to use each unit type, backed by real-world examples and best practices.

Understanding the Fundamentals

What are PX Units?

Pixels (px) are absolute units that represent a fixed size on the screen. One pixel equals one dot on the display, making it the most precise unit for exact measurements. When you set font-size: 16px, the text will always be exactly 16 pixels tall, regardless of user preferences or device settings.

What are REM Units?

REM stands for "Root EM" and is a relative unit based on the root element's font size (typically the <html> element). By default, most browsers set the root font size to 16px, so 1rem = 16px. However, this can change based on user preferences or explicit CSS declarations.

Quick Conversion Reference

Common REM to PX:

0.5rem = 8px
0.75rem = 12px
1rem = 16px
1.25rem = 20px
1.5rem = 24px

Common PX to REM:

8px = 0.5rem
12px = 0.75rem
16px = 1rem
20px = 1.25rem
24px = 1.5rem

When to Use PX Units

Perfect for Fixed Elements

Use pixels when you need precise, unchanging measurements:

Borders: border: 1px solid #ccc
Box shadows: box-shadow: 0 2px 4px rgba(0,0,0,0.1)
Small decorative elements: Icons, dividers, small spacing
Image dimensions: When you need exact pixel control

Browser Compatibility

Pixels have universal support across all browsers and devices. They're the most predictable unit, making them ideal for elements where consistency is crucial.

When to Use REM Units

Typography and Scalability

REM units excel in scenarios where scalability and user preferences matter:

Font sizes: Respects user's browser font size settings
Spacing and margins: Scales proportionally with text
Component dimensions: Maintains proportional relationships
Media queries: Creates truly responsive breakpoints

Accessibility Benefits

Using REM units significantly improves accessibility. When users increase their browser's default font size (common for visually impaired users), REM-based layouts scale appropriately, maintaining readability and usability.

💡 Pro Tip

Set your root font size using percentages: html { font-size: 100%; }. This respects the user's browser settings while giving you a predictable base for REM calculations.

Real-World Examples

Example 1: Button Component

.button {
  /* Use REM for scalable spacing and typography */
  padding: 0.75rem 1.5rem;
  font-size: 1rem;
  margin: 1rem 0;
  
  /* Use PX for precise visual elements */
  border: 1px solid #007bff;
  border-radius: 4px;
  box-shadow: 0 2px 4px rgba(0,0,0,0.1);
}

Example 2: Responsive Typography Scale

/* Base font size */
html { font-size: 100%; } /* 16px */

/* Typography scale using REM */
h1 { font-size: 2.5rem; }    /* 40px */
h2 { font-size: 2rem; }      /* 32px */
h3 { font-size: 1.5rem; }    /* 24px */
p  { font-size: 1rem; }      /* 16px */
small { font-size: 0.875rem; } /* 14px */

Best Practices and Guidelines

The Hybrid Approach

Most modern websites benefit from a hybrid approach:

Use REM for: Font sizes, padding, margins, and component spacing
Use PX for: Borders, shadows, small decorative elements, and precise layouts
Use EM for: Elements that should scale with their parent's font size

Common Mistakes to Avoid

❌ Don't: Use PX for all font sizes (breaks accessibility)
❌ Don't: Use REM for borders (creates inconsistent visual weight)
❌ Don't: Mix units randomly without a system
❌ Don't: Forget to test with different browser font sizes

Testing Your Implementation

Always test your designs with different browser font sizes:

Open your browser's settings
Change the default font size to 20px or 24px
Verify your layout still looks good and functions properly
Check that text remains readable and buttons are still clickable

Performance Considerations

Both REM and PX units have minimal performance impact. The choice between them should be based on design requirements and accessibility needs rather than performance concerns. Modern browsers handle both units efficiently.

However, using a consistent system (like REM for most measurements) can lead to more maintainable CSS and easier responsive design implementation.

Tools and Resources

To make working with REM and PX units easier, consider using:

Conversion tools: Like our REM to PX converter
CSS preprocessors: Sass/SCSS functions for automatic conversion
Browser dev tools: For testing different font sizes
Accessibility testing tools: To verify your implementation works for all users

Conclusion

The choice between REM and PX isn't about one being better than the other—it's about using the right tool for the right job. REM units excel at creating scalable, accessible designs that respect user preferences, while PX units provide the precision needed for visual consistency.

By understanding the strengths of each unit type and following the best practices outlined in this guide, you'll be able to create websites that are both visually appealing and accessible to all users.

Remember: great web design isn't just about how it looks on your screen—it's about creating experiences that work beautifully for everyone, regardless of their device, browser settings, or accessibility needs.