What is CSS Unit Converter?

CSS Unit Converter helps you instantly convert between the most common CSS length units: px, rem, and em. Whether you're building a responsive website or migrating from pixel-based to relative units, this tool makes the math effortless.

Why Convert CSS Units?

Modern web development favors relative units like rem and em over fixed pixels. Relative units adapt to user preferences and screen sizes, improving accessibility and responsive design. Converting between units ensures your designs look consistent while remaining flexible.

Best Practice: Using relative units (rem/em) instead of fixed pixels creates more accessible websites that respect user font-size preferences and scale better across different devices.

Understanding the Units

px (Pixels)

An absolute unit representing one device pixel. Provides precise, fixed sizing for elements like borders and shadows.

rem (Root EM)

Relative to the root element's font-size (usually the <html> tag). Ideal for consistent scaling across your entire page.

em (EM)

Relative to the parent element's font-size. Useful for component-level sizing, but be careful — em values compound when elements are nested.

Table of Contents

1. What is CSS Unit Converter?
- 1.1. Why Convert CSS Units?
- 1.2. Understanding the Units
2. How to Use CSS Unit Converter
3. Features
4. Frequently Asked Questions

How to Use CSS Unit Converter

Quick Conversion (Main Page)

Set your base font size

The default is 16px (standard browser default). Change it to match your project's root font-size.

Choose the input unit

Click px, rem, or em in the unit tabs.

Enter a value

Type any number and see all conversions update instantly.

Copy a result

Click the copy button next to any result to copy the value with its unit (e.g., "1.5rem").

Focused Conversion (Sub-Apps)

Choose a conversion pair

Select PX → REM, REM → PX, or PX → EM from the sub-app tabs.

Enter a value in either field

Both fields are inputs, so you can type in the "from" or "to" field.

Swap direction

Click the swap button to reverse the conversion (navigates to the opposite sub-app).

Customize Settings

Base Font Size

Affects rem calculations. Set to your project's html { font-size: ... } value.

Parent Font Size

Shown for em conversions. Set to the font-size of the parent element in your CSS context.

Decimal Places

Control output precision from 1 to 6 decimal places.

Auto-Save: All settings are saved automatically and shared between the main page and sub-apps.

Features

Core Conversion

Real-time results — Values update instantly as you type, no need to click a button.
Three CSS units — Convert between px, rem, and em in any direction.
Bidirectional input — In sub-apps, type in either field and the other updates automatically.
Customizable base size — Set your root or parent font-size for accurate project-specific conversions.

Productivity

One-click copy

Copy any result with its unit suffix (e.g., "1.5rem") ready to paste into your CSS.

Quick examples

Click common values like 12px, 16px, or 1.5rem to see instant results.

Reference table

Expandable table showing conversions for 18 common pixel values (8px–96px).

Swap conversion

Quickly reverse direction between sub-apps while keeping your current value.

Settings & Persistence

Adjustable precision — Choose 1 to 6 decimal places for output values.
Saved preferences — Base font size and decimal settings persist across sessions via localStorage.
Shared settings — Settings sync between the main converter and all sub-apps.

Privacy

100% Private: All calculations happen locally in your browser. No values are sent to any server, and no data is collected.

Frequently Asked Questions

What is the default base font size?

Most browsers use 16px as the default font-size for the root <html> element. This tool defaults to 16px, but you can change it to match your project's settings.

What is the difference between rem and em?

rem is always relative to the root element's font-size, making it consistent across your entire page. em is relative to the parent element's font-size, which means em values compound when elements are nested (e.g., 1.2em inside 1.2em equals 1.44× the root size).

rem

Root-Relative

Always relative to <html>
Consistent across page
No compounding
Best for global scaling

Parent-Relative

Relative to parent element
Context-dependent
Values compound when nested
Best for component-level sizing

When should I use rem instead of px?

Use rem for typography, spacing, and layout dimensions when you want your design to scale with user preferences. Users who increase their browser's default font size will benefit from rem-based designs. Keep px for elements that should stay fixed, like borders and box-shadows.

Use Case	Recommended Unit	Reason
Typography	rem	Scales with user preferences
Spacing/Margins	rem	Maintains proportional layout
Borders	px	Should remain fixed
Box Shadows	px	Precise visual control needed
Component Sizing	em	Relative to component context

Why does the PX to EM converter show "Parent Font Size" instead of "Base Font Size"?

Because em is relative to the parent element's font-size, not the root. The label changes to "Parent Font Size" to remind you that the reference size depends on the element's context in the DOM tree.

Important: When using em units, always consider the nesting level of your elements. Each nested level multiplies the em value by its parent's font-size.

Are my settings saved?

Yes. Your base font size and decimal precision are saved in your browser's localStorage and shared across the main converter and all sub-apps. They persist even after closing the browser.

Is my data sent to any server?

No. All conversions are calculated locally in your browser. No data is transmitted or stored on any server.

100% client-side processing
No server communication
No data collection or tracking
Settings stored only in your browser