CSS Unit Converter (PX/EM/REM/%)

Convert between CSS units: pixels, em, rem, and percentages with configurable base font sizes.

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Default browser base size is 16px
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Common Conversions (16px base)

PixelsEm/RemPercentUse Case
10px0.62562.5%Small text
12px0.7575%Captions
14px0.87587.5%Body small
16px1100%Body text
18px1.125112.5%Large text
20px1.25125%H4
24px1.5150%H3
32px2200%H2
48px3300%H1

When to Use Each Unit

  • px: Fixed sizes, borders, shadows
  • em: Relative to parent font-size (good for component spacing)
  • rem: Relative to root font-size (consistent across the page)
  • %: Relative sizing, widths, and font-sizes
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How to Use CSS Unit Converter (PX/EM/REM/%)

  1. Set your base font size (default 16px)
  2. Enter a value in any unit field
  3. All other units update automatically
  4. Reference the conversion table
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About CSS Unit Converter (PX/EM/REM/%)

Modern CSS favors relative units (em, rem, %) over fixed pixels for accessibility and responsive design. Our converter translates between these units, helping you build flexible, scalable stylesheets.

Unit Relationships

  • Pixels (px): Fixed unit; 1px = 1 device pixel (roughly)
  • Em: Relative to parent element's font size
  • Rem: Relative to root (html) element's font size
  • Percentage: Relative to parent for most properties

Why Use Relative Units

Users who adjust browser font sizes need relative units to scale proportionally. Fixed pixel sizes ignore these preferences, creating accessibility barriers. Rem-based typography scales consistently across components. Em-based spacing maintains proportions when components resize.

Conversion Math

With a 16px base font (browser default): 24px = 1.5rem = 1.5em (at root level). Our calculator handles custom base sizes—if your root is 18px, conversions adjust accordingly. Understanding these ratios improves CSS architecture decisions.

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Frequently Asked Questions

What is the difference between px, em, rem, and %?
PX (pixels) are fixed screen units. EM is relative to parent element's font size. REM is relative to root (html) font size. % is relative to parent element. For responsive design, rem is often preferred - consistent sizing based on user's browser settings. Em is useful for component-specific scaling.
Why should I use em or rem instead of pixels?
Relative units (em, rem) improve accessibility - they respect user font size preferences and scale properly. A user who needs larger text will see your em/rem-based design scale correctly. Pixels ignore these settings. Also, rem simplifies responsive design and maintains proportional relationships.
What is the default browser font size?
Most browsers default to 16px for the root font size. So 1rem = 16px by default. Users can change this for accessibility. Our converter defaults to 16px base but lets you specify your actual root font size if different. Always design assuming users might have non-standard base sizes.
How do I convert 16px to em?
EM = pixels / parent font-size. With 16px parent: 16px / 16px = 1em. For 24px with 16px parent: 24px / 16px = 1.5em. Our converter does this instantly. Enter your pixel value and base size to get em. Remember: em compounds (nested elements multiply), which can cause unexpected results.
What is the 62.5% trick for easier rem calculations?
Setting html { font-size: 62.5% } makes 1rem = 10px (62.5% of 16px), simplifying math: 1.6rem = 16px, 2.4rem = 24px. However, this requires explicitly setting body font size and can cause accessibility issues. Many developers now prefer using 16px base with rem values like 0.875rem, 1rem, 1.25rem.
Should I use em for media queries?
Yes, em-based media queries are recommended. They respect user zoom and font settings. A user who zooms 200% will trigger your breakpoints appropriately. Using px media queries ignores zoom. Convert your pixel breakpoints: 768px / 16 = 48em. Our tool helps with these conversions.