REM to PX Converter (Free AI Tool)
Convert REM to pixels and pixels to REM with bidirectional calculation. Uses simple formula: pixels = rem × base font size. Default base is 16px following W3C CSS specifications. Essential for responsive web design, accessibility compliance, and converting between absolute and relative CSS units.
Quick Reference
How It Works
- Enter REM or Pixel Value: Input the value you want to convert. The tool supports decimal values for precise calculations (e.g., 1.5rem, 24.5px).
- Set Base Font Size: Specify the root font size in pixels (default 16px). This corresponds to the font-size set on the html or :root element in your CSS.
- Instant Calculation: The converter uses the formula: pixels = rem × base font size, or rem = pixels ÷ base font size. Results update in real-time as you type.
- Use in CSS: Copy the converted value directly into your stylesheets. The tool helps maintain consistent spacing and typography scales across responsive breakpoints.
REM vs PX: Unit Comparison
| Feature | REM Units | Pixel Units |
|---|---|---|
| Type | Relative unit (scales with root font size) | Absolute unit (fixed size) |
| Accessibility | Respects user browser font size settings | Ignores user preferences |
| Responsive Design | Scales automatically across breakpoints | Requires manual adjustment per breakpoint |
| Calculation | Relative to html/root element only | Direct pixel measurement |
| Browser Support | IE9+ (97%+ global support) | Universal support |
Conversion Examples
Example 1: Typography Scale with 16px Base
0.75rem (small text)
1rem (body text)
1.25rem (large text)
1.5rem (h3 heading)
2rem (h2 heading)
2.5rem (h1 heading)12px (small text)
16px (body text)
20px (large text)
24px (h3 heading)
32px (h2 heading)
40px (h1 heading)Key Changes:
This typography scale demonstrates a common design system using REM units with 16px base font size. The scale follows a modular ratio where each level increases proportionally. Using REM ensures the entire typography system scales when users adjust browser font size settings, maintaining visual hierarchy and readability. If a user sets their browser to 20px base font (125% zoom for accessibility), 1rem becomes 20px and all typography scales proportionally: h1 becomes 50px instead of 40px. This automatic scaling is impossible with fixed pixel values, making REM essential for WCAG 2.1 AA accessibility compliance which requires text to be resizable up to 200% without loss of functionality.
Example 2: Spacing System with Custom 10px Base
0.4rem (4px spacing)
0.8rem (8px spacing)
1.6rem (16px spacing)
2.4rem (24px spacing)
3.2rem (32px spacing)
4.8rem (48px spacing)4px (extra small)
8px (small)
16px (medium)
24px (large)
32px (extra large)
48px (2x large)Key Changes:
Many developers use a 10px base font size (html font-size: 62.5% which makes 10px from default 16px) for easier mental math: 1.6rem = 16px, 2.4rem = 24px. This spacing system creates consistent margins, paddings, and gaps throughout the design. The 8px grid system (0.8rem increments) is popular in modern design systems (Material Design, Bootstrap 5) because it aligns well with common screen resolutions. Using REM for spacing ensures consistent proportions when the design scales across devices or when users adjust font sizes. The calculation becomes intuitive: divide desired pixels by 10 to get REM value. However, be aware that changing the root font size affects all REM-based measurements, so document your base size clearly for team consistency.
Frequently Asked Questions
Should I use REM or pixels for responsive design?
Use REM for typography, spacing, and layout dimensions to create truly responsive, accessible designs. REM units scale with the root font size, allowing users to adjust text size via browser settings (critical for WCAG 2.1 accessibility). Use pixels only for fixed elements like borders (1px border), shadows, or elements that must remain constant regardless of user preferences. Modern CSS frameworks (Tailwind CSS 3+, Bootstrap 5) default to REM for spacing and typography. The formula is simple: with 16px base, divide pixels by 16 to get REM (24px = 1.5rem). For media queries, use EM or REM instead of pixels to respect user zoom levels. A 768px breakpoint becomes 48rem (768÷16), ensuring layouts adapt when users zoom the page.
What is the difference between REM and EM units?
REM (root em) is relative to the root html element font size, while EM is relative to the parent element font size. REM provides predictable, consistent sizing because it always references the same base value. EM can compound in nested elements: if a parent has font-size: 1.5em and a child has font-size: 1.5em, the child is actually 2.25em relative to root (1.5 × 1.5). This compounding makes EM harder to maintain in complex layouts. Use REM for global spacing systems, typography scales, and layout dimensions. Use EM only for component-specific sizing that should scale with the component font size (like padding inside buttons where padding: 0.5em 1em scales with button font size). For most use cases, REM is the better choice for maintainability and predictability.
How do I set a custom base font size for REM calculations?
Set the base font size on the html or :root element in your CSS. For 16px base (browser default), no CSS needed. For 10px base (easier math), use: html font-size: 62.5% (62.5% of 16px = 10px). For 14px base, use: html font-size: 87.5%. Then all REM values calculate from this base. For example, with 10px base, 2.4rem = 24px. Document your base size in CSS comments and team guidelines. Avoid setting font-size in pixels on the root (html font-size: 10px) because it overrides user browser preferences, breaking accessibility. Using percentages preserves user settings while giving you a custom base. Some frameworks like Tailwind CSS use 16px base by default, while others like Chakra UI use 16px but provide easy configuration for custom bases.