CSS Unit Converter
Convert between px, rem, em, and other CSS units
What is CSS Unit Converter?
CSS Unit Converter is a free online tool that converts between CSS length units including px, rem, em, vw, vh, and percentages. Understanding and converting between CSS units is essential for responsive web design, accessibility, and consistent layouts. This tool lets you set a base font size and viewport dimensions, then converts values between any supported units. Whether you are migrating a pixel-based design to rem units for better accessibility, calculating viewport-relative sizes, or understanding the relationship between different CSS units, this converter makes the math instant and error-free. All processing happens in your browser with no data sent to any server.
How to Use
- Set the base font size (default: 16px).
- Enter a value and select the from/to units.
- The result is displayed in real time.
Tips & Best Practices
- Use rem for font sizes and spacing to ensure your design scales with user browser settings.
- Set the base font size to match your project's root font size (default is 16px in most browsers).
- Use vw and vh for full-screen sections and hero images that should scale with viewport size.
- Remember that 1rem = base font size (typically 16px), so 1.5rem = 24px at the default.
- When converting em units, note that em is relative to the parent element's font size, not the root.
Use Cases
Responsive Design
Convert pixel values to rem or vw units for responsive and accessible layouts.
Design-to-Code
Translate pixel measurements from design mockups to appropriate CSS units.
Accessibility
Migrate px-based styles to rem units so text scales with user preferences.
CSS Frameworks
Calculate correct rem values when customizing framework spacing and typography scales.
FAQ
What is the difference between rem and em?
rem is relative to the root element (html) font size, while em is relative to the parent element's font size.
How do I set the base font size?
You can set it in the base font size input field. The browser default is 16px.
What types of CSS units are there?
CSS units are divided into absolute units (px, pt, cm, etc.) and relative units (rem, em, %, vw, vh, etc.). Relative units are recommended for responsive design.
Is my data sent to any server?
No, all unit conversions are calculated locally in your browser and no data is transmitted externally.
Why should I use rem instead of px in responsive web design?
rem respects the user's browser default font size setting, improving accessibility and allowing proportional scaling without media queries.
How many pixels is 1pt?
At the web standard of 96 DPI, 1pt equals approximately 1.333px. Since 1 inch = 72pt = 96px, the conversion ratio is 96/72 = 1.333.
What is the difference between px, rem, and em?
px is an absolute unit. rem is relative to the root font size. em is relative to the parent element's font size.
Why use rem instead of px?
rem respects user browser font size settings, improving accessibility. px overrides user preferences.
Is my data collected?
No, all conversions happen in your browser. No data is sent to any server.
What is the default base font size?
Most browsers default to 16px. So 1rem = 16px unless the user or CSS has changed the root font size.
What are vw and vh?
vw (viewport width) and vh (viewport height) are relative to the browser window size. 1vw = 1% of viewport width.
When should I use em vs rem?
Use rem for consistent sizing relative to the root. Use em for component-specific sizing that should scale with its parent.
Can I convert percentage values?
Yes, percentage values can be converted relative to the parent element's dimensions or the base font size.