New Tool: CSS Size Generator | PictureSizes.com

What is the CSS Size Generator?

The CSS Size Generator is a free, client-side tool that helps developers quickly generate CSS width and height properties in multiple units. Instead of manually calculating conversions between pixels, rems, ems, percentages, and viewport units, you can now get instant, accurate CSS code ready to paste into your stylesheets.

This tool addresses a common frustration in web development: working with different CSS unit types. While pixels are straightforward, responsive design often requires using relative units like rems, viewport units like vw/vh, or flexible units like percentages. Converting between these units mentally or with a calculator is tedious and error-prone.

Our CSS Size Generator eliminates this friction. Enter any dimension in any unit, and get instant conversions to all other common CSS units, along with copy-paste-ready CSS declarations.

Key Features

Multi-Unit Support

The tool supports all major CSS length units:

Absolute units: px (pixels), pt (points), pc (picas), in (inches), cm (centimeters), mm (millimeters)
Relative units: rem (root em), em (em), % (percentage)
Viewport units: vw (viewport width), vh (viewport height), vmin, vmax
Special units: ch (character width), ex (x-height)

Enter a value in any unit, and the tool automatically calculates equivalent values in all other units. For relative units like rem and em, you can customize the base font size to match your project's configuration.

Instant CSS Output

Get properly formatted CSS declarations with one click. The tool generates:

Standard properties: width: 320px;
Max/min properties: max-width: 20rem;
Combined properties: width: 100%; max-width: 1200px;
Shorthand syntax: width: clamp(16rem, 50vw, 75rem);

Each output includes a copy button for instant clipboard copying.

Responsive Presets

Common breakpoint sizes are available as quick presets:

Mobile: 320px, 375px, 414px
Tablet: 768px, 834px, 1024px
Desktop: 1280px, 1440px, 1920px
Container widths: 640px, 768px, 1024px, 1280px, 1536px

Click any preset to instantly populate the calculator with that value and see all unit conversions.

Custom Base Font Size

Since rem units are relative to the root font size (usually 16px), the tool lets you set a custom base. This is essential for projects that use a different base font size or for calculating values with precision.

Common base font sizes (14px, 16px, 18px, 20px) are available as quick buttons, or you can enter any custom value.

Real-Time Preview

See exactly how your dimensions will look with a live visual preview. The preview updates instantly as you adjust values, showing actual rendered sizes for both width and height properties.

History and Favorites

The tool saves your recent calculations in browser localStorage, making it easy to reference previous values. You can also favorite specific calculations for quick access later.

How to Use It

Basic Usage

Enter a dimension value in any unit (e.g., "320px" or "20rem")
The tool automatically detects the unit and calculates conversions
Browse the output section to see your value in all other units
Click the copy button next to any CSS declaration to copy it to your clipboard
Paste the code directly into your stylesheet

Advanced Workflows

For Responsive Design: Enter a desktop width in pixels, then copy the rem or vw equivalents for responsive scaling. Use the max-width output to set flexible container sizes that adapt to screen size.

For Accessibility: Convert pixel-based designs to rem units, which respect users' browser font size preferences. This is crucial for meeting WCAG accessibility guidelines.

For Fluid Typography: Use the clamp() output to create fluid sizing that scales between minimum and maximum values based on viewport width.

Use Cases

Converting Design Mockups

Design tools like Figma and Sketch export dimensions in pixels. If your CSS framework uses rems or you're building a responsive site, you need to convert these values. The CSS Size Generator makes this instant: paste the pixel value, copy the rem equivalent.

Building Responsive Layouts

When creating layouts that adapt to different screen sizes, you often need the same dimension expressed in multiple units. For example, a sidebar might be 300px wide on desktop, but on smaller screens, you want it to be 40% of the viewport width. Use the generator to quickly calculate both values.

Maintaining Design Systems

Design systems often define spacing scales in one unit (e.g., pixels) but need to be implemented in another (e.g., rems for accessibility). The CSS Size Generator helps maintain consistency when translating design tokens into code.

Learning CSS Units

New developers often struggle to understand how different CSS units relate to each other. The generator serves as an educational tool, showing in real-time how changing a value in one unit affects all others.

Examples

Example 1: Container Width

You want a content container that's 1200 pixels wide but also accessible and responsive. Enter "1200px" and the tool generates:

width: 1200px; — Standard pixel width
width: 75rem; — Respects user font size preferences
width: 62.5vw; — Scales with viewport width (on 1920px screen)
max-width: 1200px; — Prevents overflow on large screens
width: clamp(320px, 90vw, 1200px); — Fluid width between mobile and desktop

Example 2: Responsive Spacing

Your design calls for 64px of padding on desktop. Enter "64px" to get:

padding: 64px; — Fixed spacing
padding: 4rem; — Scales with base font size
padding: 3.33vw; — Proportional to viewport (on 1920px screen)
padding: clamp(2rem, 4vw, 4rem); — Fluid between 32px and 64px

Example 3: Icon Sizes

You have icons at 24px but want them to scale for accessibility. Enter "24px":

width: 24px; height: 24px; — Fixed size
width: 1.5rem; height: 1.5rem; — Scales with user preferences
width: 1.5em; height: 1.5em; — Scales with parent font size

Why We Built It

After launching PictureSizes.com with tools for image dimensions, we heard from developers who needed help with CSS sizing too. Many existing CSS generators are cluttered with ads, require accounts, or lack the specific features developers need daily.

We wanted to create a tool that:

Works instantly: No page loads, no waiting. Pure client-side JavaScript for immediate results.
Respects privacy: All calculations happen in your browser. No data sent to servers. No tracking.
Covers real workflows: Not just unit conversion, but actual CSS output you can use in production code.
Looks good: Clean interface that matches the aesthetic of modern development tools. Dark mode support.
Stays accessible: Keyboard navigation, screen reader support, and high contrast ratios throughout.

The CSS Size Generator is built with the same philosophy as all our tools: no bloat, no tracking, just functionality that helps you work faster.

Technical Details

For the curious, here's how it works:

Built with vanilla JavaScript: No frameworks, no dependencies. Just 8KB of gzipped code.
Conversion accuracy: All calculations use standard CSS unit definitions. Viewport units assume a default 1920×1080 reference but update based on your actual viewport.
Base font size defaults: Follows browser standards (16px) but lets you customize for your project.
LocalStorage: History and favorites are stored locally in your browser, not on our servers.
Copy mechanism: Uses the modern Clipboard API with fallback for older browsers.

What's Next

This is version 1.0 of the CSS Size Generator, and we have plans to expand it based on user feedback:

Planned Features

CSS Custom Properties: Generate CSS variables for your sizes to use in design systems
Sass/SCSS Output: Export as Sass variables or functions
Bulk Conversion: Paste multiple values and convert them all at once
Tailwind Config: Generate Tailwind config values for custom spacing scales
Grid Calculations: Tools for CSS Grid track sizing and gap calculations
Flexbox Helpers: Calculate flex-basis and other flex properties

Integration Plans

We're exploring integrations with popular code editors. Imagine highlighting a pixel value in VS Code and getting a context menu to instantly convert it to rems or generate a fluid clamp() function. Stay tuned.

Try It Now

The CSS Size Generator is live and ready to use. No account required, no installation needed. Just visit the tool and start generating CSS.

Open CSS Size Generator →

Related Resources

All Tools - Browse our complete collection of image and CSS tools
Responsive Breakpoint Calculator - Calculate media query breakpoints
Responsive Images Guide - Learn about responsive design principles
WebP vs AVIF Comparison - Optimize your image formats

Introducing Our New CSS Size Generator