CSS Unit Converters (PX, REM, EM)

CSS Units for Responsive Web Design

CSS offers multiple unit types for sizing elements. Choosing the right unit affects how your designs respond to different screen sizes and user preferences like font scaling. Understanding these units is fundamental to accessible, responsive web development.

Absolute Units

Pixels (px) are the most common absolute unit. A CSS pixel doesn't always equal a physical pixel—on high-DPI displays, one CSS pixel may span multiple physical pixels. Pixels provide precise control but don't scale with user font preferences.

Relative Units

REM (root em) is relative to the root element's font size (typically 16px by default). If a user increases their browser's base font size, rem-based layouts scale proportionally. This makes rem ideal for accessible designs. Conversion: 1rem = 16px (at default settings).

EM is relative to the parent element's font size. This creates compounding effects—nested elements multiply em values. While powerful for component-based scaling, ems can become unpredictable in deeply nested structures.

Viewport units (vw, vh) are relative to browser window dimensions. 1vw equals 1% of viewport width. These are useful for full-screen layouts and fluid typography but can cause issues on mobile devices with dynamic viewport heights.

When to Use Each Unit

• REM: Typography, spacing, component sizing—anything that should scale with user preferences
• PX: Borders, shadows, fine details where precise control matters
• EM: Component-internal spacing that should scale with the component's font size
• VW/VH: Hero sections, full-bleed layouts, fluid typography

Accessibility Considerations

Using rem instead of px for font sizes allows users who need larger text to scale content with their browser settings. WCAG guidelines recommend layouts that work at 200% zoom—rem-based designs achieve this naturally.