What units does this converter support?

This tool converts between CSS units (px, rem, em, vw, vh, %), length units (mm, cm, in, pt), and common developer units. It handles the relationships between absolute and relative units based on your specified base values.

What is the difference between em and rem?

Both are relative units, but em is relative to the parent element's font size, while rem is relative to the root (html) font size. Rem is more predictable because it doesn't compound — nested elements with em sizing can produce unexpected results.

When should I use px vs rem?

Use rem for font sizes, spacing, and layout dimensions — they scale with user accessibility settings. Use px for borders, shadows, and cases where you need exact pixel values. Use em for component-level scaling where children should inherit parent sizing.

CSS Unit Converter — px to rem

What Is a Unit Converter?

This unit converter handles CSS and design units — px, rem, em, vw, vh, percentages, and physical units (mm, cm, in, pt). Understanding the relationships between these units is essential for responsive design, accessible typography, and consistent spacing systems.

CSS Unit Guide

px — fixed, absolute pixels. rem — relative to root font size (default 16px). em — relative to parent's font size (compounds in nested elements). vw/vh — percentage of viewport width/height. % — percentage of parent element. pt — points (1pt = 1/72 inch), used in print. Use our PX to REM converter for bulk CSS conversions.

How to Use This Unit Converter

Enter a value — Type the number you want to convert in the input field.
Select the source unit — Choose the CSS unit you're converting from — px, rem, em, vw, vh, %, pt, cm, or mm.
Select the target unit — Choose the unit you want to convert to. The tool calculates the equivalent value instantly.
Set the base font size — For rem and em conversions, specify the root font size (typically 16px) to ensure accurate results.

Tips and Best Practices

→ Use rem for most sizing. Rem units scale with the user's browser font-size preference, making your design accessible. Use px only for borders, shadows, and decorative details.
→ 1rem = 16px by default. Unless you've changed the root font-size, the browser default is 16px. So 1rem = 16px, 0.5rem = 8px, 2rem = 32px.
→ Use vw/vh sparingly. Viewport units are great for full-screen layouts, but text sized in vw can become unreadably small on mobile or huge on ultra-wide monitors. Combine with clamp() for safety.
→ Know when to use em vs rem. Em is relative to the parent element's font-size (compounds when nested). Rem is relative to the root (<html>) font-size (consistent everywhere). Prefer rem for predictability.

Frequently Asked Questions

What units does this support?

CSS units (px, rem, em, vw, vh, %), length units (mm, cm, in, pt), and common developer units. Handles relationships between absolute and relative units.

em vs rem?

Both are relative, but em is relative to the parent's font size (compounds in nesting), while rem is relative to the root font size (predictable, doesn't compound).

px vs rem?

Use rem for font sizes, spacing, and layout (scales with accessibility settings). Use px for borders, shadows, and exact-pixel needs. Use em for component-level scaling.

What is the difference between px, em, and rem?+

Pixels (px) are absolute units — 1px is always 1px regardless of context. Em is relative to the parent element's font size, so it compounds when nested. Rem is relative to the root (html) element's font size, making it consistent throughout the page. For most use cases, rem is the best choice.

How do I convert px to rem?+

Divide the pixel value by the root font size (usually 16px). For example: 24px ÷ 16 = 1.5rem. Our converter handles this math automatically for any base font size.

When should I use viewport units (vw, vh)?+

Viewport units are ideal for full-screen sections (height: 100vh), responsive spacing that scales with screen size, and fluid typography (combined with clamp). Avoid using vw/vh alone for font sizes, as they can become too small on mobile or too large on desktop.

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Built by Derek Giordano · Part of Ultimate Design Tools

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