What is the difference between rem and em in CSS?

rem (root em) is always relative to the root HTML element's font size (usually 16px by default). em is relative to the current element's parent font size, which can compound unpredictably when elements are nested.

How do I convert px to rem?

Divide the pixel value by your root font size. With the default 16px root: 24px ÷ 16 = 1.5rem. If you have changed your root font size, adjust the divisor accordingly.

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Pixels to REM/EM Converter

Convert CSS pixel values to REM or EM units based on a configurable root font size.

⚡ Instant 🆓 Free 🔒 Private

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Root Font Size (px)

Usually 16px in browsers. Change if you've set a custom root font size.

Pixels (px)

REM

EM (relative to parent)

💡 Tip: REM is always relative to the root font size. EM is relative to the current element's parent. For responsive CSS, prefer rem.

CSS Units: px, rem, and em

CSS offers multiple units for specifying dimensions. Understanding the difference between px, rem, and em is essential for building accessible, responsive websites.

Unit Comparison

px (Pixels): An absolute, fixed unit. 1px is always 1 pixel regardless of parent or root font size.

rem (Root EM): A relative unit based on the root element's (<html>) font size. If the root is 16px, then 1rem = 16px exactly.

em: A relative unit based on the current element's parent font size. This makes it context-dependent and can cause cascading complexity.

Why Use rem?

Using rem for font sizes and spacing makes your entire design scale proportionally when a user changes their browser font preference. This is critical for web accessibility. rem avoids the cascading nesting issues of em while still being responsive to user preferences.