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Enter a value and choose a direction to convert between pixels, rem, and em units.
What Are REM and EM Units?
In CSS, rem (root em) and em are relative length units that scale with font size. Unlike pixels (px), which are absolute, rem and em adapt to the user's preferences and parent context — making them essential for accessible, responsive design.
A rem unit is always relative to the root element's font size — the <html> tag. In most browsers, the default root font size is 16px. So 1rem = 16px, 2rem = 32px, and so on. Changing the root font size in your CSS instantly scales every rem-based value across the entire page.
An em unit is relative to the font size of its parent element. If a parent has font-size: 20px;, then 1em = 20px for that child. This compounding behavior makes em powerful for component-level scaling but can become unpredictable in deeply nested layouts.
Why Convert PX to REM?
Modern frontend development strongly favors rem over px for several practical reasons:
- Accessibility compliance: Users who increase their browser's default font size expect your site to scale proportionally. Rem units respect this setting; px units do not.
- Maintainability: Changing one root font-size rule updates the entire typographic scale, spacing system, and component sizing — no find-and-replace across hundreds of declarations.
- Consistency: A rem-based design system creates a predictable mathematical relationship between all sizes, making handoffs between designers and developers smoother.
- Responsive scaling: Rem pairs naturally with CSS
clamp()and viewport units for fluid, resolution-independent interfaces.
How the Conversion Works
The math is straightforward. Divide or multiply by your base font size:
px → rem = pixels ÷ base font size
rem → px = rem × base font size
px → em = pixels ÷ parent font size
em → px = em × parent font size
Example: Converting 24px to rem with a 16px base:
24px ÷ 16px = 1.5rem
Most design systems choose a default base of 16px because it aligns with browser defaults and makes mental math easier: 8px = 0.5rem, 16px = 1rem, 32px = 2rem.
Common Base Font Sizes by Framework
Different CSS frameworks and projects set different root font sizes. Here are the most common defaults.
| Framework / Project | Root Font Size | Notes |
|---|---|---|
| Browser Default | 16px | Standard across all major browsers |
| Tailwind CSS | 16px | Uses rem by default in the spacing scale |
| Bootstrap 5 | 16px | $font-size-root variable |
| Material Design | 16px | 1rem = 16sp (scalable pixels) |
Frequently Asked Questions
What is the difference between rem and em?
rem is always relative to the root (<html>) font size, while em is relative to the parent element's font size. rem is generally preferred for font sizes and layout spacing because it avoids compounding issues in nested components.
Should I use rem or em for padding and margins?
Most developers use rem for padding, margins, and layout spacing because it stays consistent regardless of nesting depth. em can be useful when you want spacing to scale with a component's own font size — for example, button padding that grows with the button text.
Why is 16px the default base font size?
16px has been the browser default since the early days of the web. It provides comfortable readability for body text at normal viewing distances. Users can change this in browser settings, which is why rem-based designs are more accessible.
Can I change the root font size to make rem math easier?
Yes — some developers set html { font-size: 62.5%; } to make 1rem = 10px. This simplifies mental math (1.6rem = 16px) but can interfere with user accessibility settings. The modern recommendation is to keep 1rem = 16px and use a calculator or design-token system.
Does this converter work with CSS clamp() values?
This tool converts single values between px, rem, and em. For fluid typography that scales between viewport widths, use our CSS clamp() Calculator which generates the full clamp(min, preferred, max) expression.
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