Px / Em / Rem Converter
Convert between px, em, and rem values for CSS sizing. Perfect for responsive design calculations.
Input Values & Settings
Base Settings
Converter
Conversion Results
CSS Code
font-size: 16px;font-size: 1em;font-size: 1rem;Visual Preview
font-size: 16px;font-size: 1em;font-size: 1rem;Understanding CSS Units
About Px / Em / Rem Converter
A px/em/rem converter is an essential CSS utility tool that accurately converts between different CSS unit measurements including pixels (px), em units, and rem units, taking into account base font sizes and parent element relationships. This tool helps developers create responsive designs by providing precise calculations for different CSS sizing units used in modern web development.
Why use a Px / Em / Rem Converter?
Using a px/em/rem converter ensures consistent and scalable typography and spacing across responsive designs, helps maintain accessibility by using relative units appropriately, and saves time by eliminating manual calculations when converting between CSS units. It prevents sizing errors in responsive layouts, supports better user experience with scalable designs, and ensures proper implementation of modern CSS best practices.
Who is it for?
This tool is indispensable for front-end developers building responsive websites with scalable typography, UI/UX designers creating accessible and flexible design systems, CSS developers implementing modern layout techniques, web developers converting legacy pixel-based designs to responsive units, and anyone working with CSS who needs accurate conversions between different measurement units for optimal responsive design implementation.
How to use the tool
Enter the value you want to convert in the source unit field (px, em, or rem)
Specify the base font size if needed for accurate em and rem calculations
Select the target unit you want to convert to from the available options
View the automatically calculated equivalent values in all supported units
Copy the converted values for use in your CSS stylesheets and responsive designs
Frequently Asked Questions
How do I convert between px, em, and rem?
Enter a value in any unit and the tool converts to the other two. Configurable base font size (default 16px — the browser standard). Conversions: 1rem = base size (16px by default); 1em = relative to parent element's font size (depends on context). Runs entirely in your browser — values never leave the device. Useful for: responsive web design, accessibility-aware sizing, migrating from px-based to rem-based stylesheets.
What's the difference between px, em, and rem?
**px** (pixels): absolute unit. 16px is 16 pixels — same size everywhere, regardless of context. **em**: relative to the FONT SIZE OF THE PARENT element. If parent is 20px, 1em = 20px; 0.5em = 10px. **rem** (root em): relative to the FONT SIZE OF THE ROOT (`<html>`) element. By default, the root is 16px, so 1rem = 16px; 2rem = 32px. The key difference: em compounds with parents (nested ems multiply); rem is always relative to the root.
Is my data sent to a server?
No — conversion runs entirely in your browser via JavaScript arithmetic. Your values never reach a server, never get logged. Verify in DevTools' Network tab: zero HTTP requests during conversion. The math is deterministic; no server processing needed.
Why use rem instead of px?
**Accessibility**. Users with low vision can change the browser's default font size (Settings > Appearance > Font Size in most browsers). With rem-based sizing, the whole interface scales proportionally — text, spacing, components. With px, only browser-zoom scales the page (which scales everything including images, potentially breaking layouts). **Best practice**: rem for typography and spacing (padding, margins, gaps); px for borders and shadows (small, fixed values where pixel precision matters); em for component-local sizing (e.g., button's padding relative to its own font-size).
What's the em compounding problem?
When elements nest, em compounds: outer element has font-size 1.5em (24px from 16px base); inner element has font-size 1.5em — but now relative to PARENT (24px), so inner = 36px. Three levels of 1.5em: 16 * 1.5 * 1.5 * 1.5 = 54px. Surprising for deeply-nested components. Rem avoids this: rem is ALWAYS relative to root (16px), no compounding. Use em deliberately (intentional contextual sizing — e.g., button's padding-x = 1em scales with button's font size). Use rem by default to avoid surprise.
What's the default browser font size and why 16px?
Default in all major browsers: **16px**. Set in the browser's user-agent stylesheet. Users can change this (Settings > Appearance). Why 16px historically: dates to early Macintosh and Windows fonts where 16px was the standard reading size. Modern designs work with this baseline. **Don't override the root font-size to '10px for easy math'** — this defeats user font-size preferences. If you want '1.6rem = 16px' for math simplicity, accept that users who change their default font size lose that mapping. Best practice: leave root at the default; use values like `0.875rem` (14px) and `1.25rem` (20px) directly.
Should I use clamp() or fluid sizing?
Modern responsive sizing: **`clamp(min, preferred, max)`** scales smoothly. Example: `font-size: clamp(1rem, 2.5vw + 1rem, 1.75rem)` — text scales fluidly with viewport width, bounded between 1rem (16px) and 1.75rem (28px). Eliminates breakpoint-jumping (sudden size changes at media query boundaries). For type scales, fluid sizing is the modern best practice. Supported in all modern browsers (since 2020). For older browser support, use traditional media queries with stepped rem values. For most new projects in 2026, clamp() is the right default.
When is px the right choice?
Specific cases. (1) **Borders**: `border: 1px solid` — pixel precision matters; 0.0625rem looks the same but is wasteful. (2) **Box-shadows**: typically pixel-defined for design consistency. (3) **Media query breakpoints**: traditionally in px (though em-based breakpoints have advantages — they respect user font-size). (4) **SVG/image sizes**: when pinning to specific pixel dimensions. For typography, padding, margins, gap, line-height: use rem. The rule of thumb: 'does this need to scale with user font-size preference?' — yes = rem; no = px.
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