How It Works

PX (pixels) and REM (Root EM) are two important length units in CSS. PX is an absolute unit, while REM is relative to the root element (<html>) font size.

Conversion Formula

PX → REM: REM = PX ÷ Root Font Size

REM → PX: PX = REM × Root Font Size

Example: With 16px root, 32px = 2rem, 1.5rem = 24px

Why Use REM?

Responsive Design: Simply adjust root font size to scale the entire page proportionally
User Accessibility: Respect user browser font size preferences (WCAG compliance)
Easy Maintenance: Unified scaling ratio reduces media query complexity
Better Readability: Text and layout maintain harmonious proportional relationships

Use Cases

Responsive Layout

Use unified REM values across mobile, tablet, and desktop, achieving perfect adaptation by adjusting root font size.

Accessibility Design

Follow WCAG standards, allowing visually impaired users to adjust browser font size without breaking layout.

Component Library

Build reusable UI components with REM to ensure flexibility across different projects.

Design Conversion

Quickly convert PX values from Figma, Sketch, and other design tools to REM for development.

CSS Framework Migration

Batch convert PX units in legacy projects to REM, improving code modernization and maintainability.

Theme System

Set different root font sizes for multi-theme websites to implement "small/medium/large" font mode switching.

Frequently Asked Questions

What's 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. REM is more predictable and easier to maintain, avoiding compound calculation issues from nested levels.

What is the browser's default root font size?

Most modern browsers have a default root font size of 16px. However, users can modify this value in their browser settings, and using REM respects user personalization preferences.

How do I set the root font size in my project?

Set font-size on the html element in CSS. For example: html { font-size: 16px; }. Mobile projects often use font-size: 62.5%; (10px) for easier calculations.

Should all CSS properties use REM?

Not necessarily. Font sizes, padding, and margins work well with REM. Borders typically use PX (1px borders are more precise). Media query breakpoints are also recommended to use EM or PX.

Is REM compatible with all browsers?

REM units are supported in IE9+ and all modern browsers. If IE8 compatibility is required, you can use PX as a fallback (CSS Polyfill).

Does using REM affect performance?

No. Browsers convert REM to absolute values (PX) during rendering, and the performance difference is negligible. Modern rendering engines are well-optimized for relative units.

What's the principle behind mobile REM adaptation?

The common approach uses JavaScript to dynamically set root font size based on screen width. For example: document.documentElement.style.fontSize = (window.innerWidth / 375) * 16 + 'px';. This achieves proportional scaling.

Why do people recommend the 62.5% technique?

Setting html { font-size: 62.5%; } makes 1rem = 10px (based on 16px default). This makes calculations more intuitive: 14px = 1.4rem, 24px = 2.4rem. But remember to reset body font size to 1.6rem (16px).

Are decimal REM values (like 0.875rem) safe?

Completely safe. Modern browsers support high-precision REM calculations. However, some older browsers may have 1px rounding errors. If pixel-perfect precision is required, use PX for critical positions.

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PX ↔ REM Converter

PX → REM

REM → PX

Quick Reference (Root: px)