PX vs REM vs EM: Which CSS Unit Should You Use?
Choosing the right CSS unit can make or break your responsive design. Pixels, REMs and EMs each behave differently, and using the wrong one leads to accessibility issues and layouts that break on different screen sizes. Here is when to use each one.
Pixels (px): The Absolute Unit
Pixels are fixed-size units. 16px is always 16 pixels regardless of any other setting. This makes them predictable but inflexible.
The problem: When a user changes their browser default font size (for accessibility), pixel-based text does not scale. This affects millions of users who rely on larger text to read comfortably.
When to use px: Borders, box shadows, and small decorative details where exact sizing matters and scaling is not needed.
REM: Relative to the Root
REM stands for "root em." It is relative to the root element (<html>) font size. Most browsers default to 16px, so 1rem = 16px, 1.5rem = 24px, 0.875rem = 14px.
The key advantage: if a user changes their default font size to 20px, all your REM-based values scale proportionally. Your layout adapts automatically.
When to use rem: Font sizes, spacing (margin, padding), media queries, max-widths. REM should be your default unit for most properties.
EM: Relative to the Parent
EM is relative to the font size of the parent element, not the root. This creates a compounding effect: if a parent is 1.2em and a child is also 1.2em, the child ends up at 1.44em of the root. This compounding can quickly get out of control.
When to use em: Padding and margins on components that should scale with their own font size (like buttons), icon sizing relative to text.
Quick Comparison
| Feature | px | rem | em |
|---|---|---|---|
| Relative to | Nothing (absolute) | Root font size | Parent font size |
| Scales with user settings | No | Yes | Yes |
| Compounds | No | No | Yes |
| Best for | Borders, shadows | Most things | Component-local scaling |
The Simple Rule
Use REM for almost everything. Use px only for borders, outlines, and box shadows. Use em only when you intentionally want something to scale relative to its parent font size. This approach gives you accessible, responsive designs with minimal effort. Modern frameworks like Tailwind CSS follow this exact philosophy.
Convert PX to REM instantly
Use our converter to quickly translate pixel values to REM with any base font size.
Open PX to REM Converter