PX to REM Converter
Convert pixel values to scalable REM units for responsive, accessible CSS. Bulk convert with a custom base font size.
Why Use REM Instead of PX?
REM (root em) units are relative to the root element font size, making your design scalable and accessible. When users change their browser font size, REM-based layouts adapt automatically. The standard base font size is 16px, so 1rem = 16px by default. Use this tool to bulk-convert your design tokens from pixels to REM.
PX vs REM: Why REM Units Are Better for Modern Web Design
The choice between pixel units and REM units in CSS is more than a stylistic preference — it has direct implications for accessibility, responsive design, maintainability, and user experience. While pixels offer predictability and precision, REM units unlock scalability and respect user preferences in ways that pixels fundamentally cannot. Understanding when and why to use each unit type — and how to convert between them efficiently — is an essential skill for any frontend developer building production websites.
What Is the Difference Between PX, EM, and REM?
PX (pixels) are absolute units — 16px is always 16 pixels regardless of context, user settings, or parent element. This predictability is useful for things that should never change size regardless of user preferences: border widths, shadows, fine-grained decorative details.
EM is a relative unit that is relative to the parent element's font size. This creates a compounding effect — an element nested inside parents that each have font-size: 1.2em will have its font size multiplied at each level, quickly becoming unpredictably large. EM is still useful for padding and margins that should scale with an element's own font size, but it requires careful management in nested contexts.
REM (root em) is always relative to the root element (<html>) font size, regardless of nesting depth. The default root font size in browsers is 16px, so 1rem = 16px, 1.5rem = 24px, 0.875rem = 14px. Because REM always references the root, it is predictable and free from the compounding issues of EM, while still being relative and scalable.
Accessibility: The Most Important Reason to Use REM
Many users — particularly those with low vision — increase their browser's default font size through browser settings. Chrome, Firefox, and Safari all allow users to set a default font size larger than 16px. When you use pixel units for font sizes, these user preferences are completely ignored — the text remains exactly as you specified in pixels, overriding the user's accessibility settings. When you use REM units, the user's root font size preference is respected and all REM-based sizes scale proportionally, making your content accessible. This is the single most important argument for using REM for typography.
The 62.5% Trick
Many developers use a technique of setting the root font size to 62.5% (html { font-size: 62.5%; }), which makes the default root font size 10px instead of 16px. This means 1rem = 10px, making mental arithmetic much easier — 1.4rem = 14px, 2.4rem = 24px, and so on. If you use this technique, remember to reset the body font size to 1.6rem (16px equivalent) so text doesn't appear too small. Our converter supports any base font size — simply change the base value to 10 when using the 62.5% technique.
What Should Use REM and What Should Use PX?
Use REM for: font sizes (critical for accessibility), line-height values, spacing (padding, margin, gap) that should scale with text size, max-width and container widths, and component dimensions that should scale with the design. Use PX for: border widths (1px borders should remain 1px), box shadows, border-radius on very small elements, and other fine-grained details where sub-pixel precision matters. Media query breakpoints are a special case — em units in media queries are generally preferred over both px and rem for technical reasons related to how browsers handle user zoom.
Converting Design Tokens from PX to REM
When working from a Figma or Sketch design file where everything is specified in pixels, you need to convert all typography and spacing values to REM before implementing. This is exactly what this converter is built for — paste your full list of pixel values (one per line), set your base font size (typically 16), and instantly get the REM equivalents. Copy the results directly into your CSS custom properties or design token file.
REM in Design Systems
Production design systems from companies like Shopify (Polaris), GitHub (Primer), and Atlassian (Atlassian Design System) all use REM units for their typography and spacing scales. Tailwind CSS's default spacing and typography scales use REM units throughout. Adopting REM early in a project aligns you with industry best practices and makes your codebase compatible with component libraries and design tokens that assume REM as the standard unit.