Pixel ↔ REM Converter
Convert between pixels and REM units for CSS
How to use Pixel ↔ REM Converter
Convert between pixels and REM units for CSS. Set your base font size. Instant conversion. Free online pixel to REM calculator.
Why do web developers convert between pixels and REM?
Modern CSS best practice favors relative units (REM, EM) over absolute pixels for font sizes and spacing. This ensures that users who set a larger default font size in their browser preferences see appropriately scaled text — a core accessibility requirement.
- Font sizing: Instead of
font-size: 16px, usefont-size: 1rem. If the user's browser base font is 20px, everything scales proportionally. - Spacing and padding: Margins and padding in REM scale with the user's font preference.
padding: 1.5remis more accessible thanpadding: 24px. - Design system implementation: Design tools (Figma, Sketch) output values in pixels. Convert to REM before writing CSS so the design system is accessible by default.
- Media queries: REM-based media queries respect the user's font size preference.
@media (min-width: 48rem)triggers at 768px for a 16px base, but at a higher pixel value for users with larger fonts. - Component libraries: Building reusable components in REM ensures they behave consistently across different base font size contexts.
REM vs EM: REM (Root EM) is always relative to the root element (html) font size — consistent and predictable. EM is relative to the current element's font size — can compound unexpectedly in nested elements. REM is preferred for most use cases.
Frequently Asked Questions
What is the default browser font size?
The default is 16px in all major browsers (Chrome, Firefox, Safari, Edge). Users can change this in browser settings — this is the whole point of using REM. 1rem = 16px by default, but could be 18px, 20px, or even 12px depending on user preferences.
Why do some developers set html { font-size: 62.5% }?
Setting the root font size to 62.5% of 16px = 10px makes the math easy: 1.6rem = 16px, 2.4rem = 24px. This is a common trick but it overrides the user's preferred font size, partially defeating the accessibility purpose of using REM. Modern CSS calc() is a better alternative.
What is the difference between REM, EM, and PX?
PX is an absolute unit — always the same size regardless of settings. EM is relative to the parent element's font size — compounds in nested elements. REM is relative to the root (html) font size — consistent throughout the document. For accessible design, prefer REM over PX for font sizes and spacing.
When should I use PX instead of REM?
Use PX for borders (1px borders should always be 1px), box shadows, and elements where the exact pixel size matters regardless of font preferences — like a 2px separator line. Use REM for font sizes, padding, margin, and any spacing that should scale with text.
How do I convert a whole design from pixels to REM?
Divide every pixel value by the root font size (default: 16). So 24px ÷ 16 = 1.5rem. Tools like Sass functions (px-to-rem()) or PostCSS plugins can automate this conversion across a codebase.
REM vs EM vs PX vs VW/VH vs percentages
PX is absolute — precise but not scalable. REM scales with the root font size — best for typography and spacing. EM scales with the parent font size — useful for component-level spacing but can compound. VW/VH are percentages of the viewport — useful for fluid layouts and full-screen sections. Percentages are relative to the parent element — essential for responsive widths. Modern CSS uses a combination: REM for type and spacing, VW/VH for layout, percentages for widths, and PX only for fine details like borders.