PX to REM Converter
Convert pixels to REM and vice versa.
PX to REM: Complete Guide to CSS Units
REM (root em) is a unit relative to the font size of the root element (html). By default, browsers set the root font-size to 16px, so 1rem = 16px. Unlike EM (which is relative to the parent), REM always refers to the root, ensuring predictable and consistent behavior throughout the application.
Using REM instead of PX significantly improves your website's accessibility. When a user changes the base font size in their browser (for visual needs or preferences), all elements defined in REM scale proportionally, while PX elements remain fixed and ignore the user's preference.
Practical Use Cases
Responsive typography: Define all font-sizes in REM so text scales uniformly when the user modifies the base size. Use PX only for elements that must maintain a fixed size (borders, shadows).
Consistent spacing: Use REM for margin, padding, and gap. A REM-based spacing system (0.25rem, 0.5rem, 1rem, 1.5rem, 2rem) scales proportionally with typography.
Design systems: Frameworks like Tailwind CSS use REM extensively. Understanding the PX/REM conversion is essential for customizing and extending these design systems.
Frequently Asked Questions
What is the difference between PX, EM, and REM?
PX is absolute (always the same size). EM is relative to the parent element's font-size (compounds in nested elements). REM is relative to the root font-size (always predictable). For most cases, REM is the best choice.
Should I change the root font-size to 62.5%?
Some developers set html { font-size: 62.5% } to make 1rem = 10px, making mental calculations easier. However, this can break accessibility if you don't use percentages. The recommended alternative is html { font-size: 62.5% } only if you define all sizes in REM.
When should I use PX instead of REM?
Use PX for: borders (1px solid), shadows (box-shadow), media queries (although some prefer em), and elements that must maintain an exact size regardless of user settings (such as fixed-size SVG icons).
What base font-size does this tool use?
The default value is 16px, which is the standard for all modern browsers. You can change it if your project uses a different base (for example, if your CSS reset sets a different root font-size).
CSS Units: When to Use Each One
| Unit | Type | Best for |
|---|---|---|
| rem | Relative to root | Typography, spacing |
| em | Relative to parent | Self-contained components |
| px | Absolute | Borders, shadows |
| % | Relative to parent | Fluid layouts |
| vw/vh | Relative to viewport | Hero sections, fullscreen |