How is the root font size used?

It's the base for rem, em, and percent conversions, matching your project's html font-size.

What's the viewport value for?

It's used to convert vw and vh units to and from pixels.

Are em and rem treated the same?

For simplicity both are converted relative to the root font size; nested em contexts aren't modeled.

CSS Unit Converter

Convert between px, rem, em, %, pt, vw, and vh.

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Root font size (px)

Viewport width/height (px)

Value to convert

Value

Unit

Conversions

rem

100

1.1111

em and % are treated relative to the root font size for simplicity; vw and vh use the same viewport dimension entered above.

About the CSS Unit Converter

Convert a CSS length between pixels, rem, em, percent, points, vw, and vh, with configurable root font size and viewport dimension so the conversions match your actual project setup.

Examples

16px to rem

16px, root 16px

Output

1rem

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Frequently asked questions

Version 1.0.0 · Updated 2026-08-06 · Runs entirely in your browser