PX to REM Converter: Responsive CSS Unit Conversion for Modern Web Design
The shift from px to rem is one of the most important practices in modern CSS. Pixel values are absolute and ignore user preferences, while rem values scale with the browser's root font size — making your typography and layout accessible to users who rely on larger text for readability.
Our converter supports single values, bulk conversion with common design system presets (Tailwind, spacing scales, type scales), and customizable root/parent sizes for any design system.
The core conversion formula:
When to Use rem vs em vs px
Use rem for: font-size (body, headings, labels), max-width containers, spacing between sections
Use em for: padding/margin within a component (scales with its font-size), border-radius on buttons, line-height
Use px for: borders (border: 1px solid), box-shadow blur radius, fine-grained pixel-perfect details, media query breakpoints (debated)
Design System Conversions (Tailwind CSS)
Tailwind CSS uses a 4px base spacing grid. With 16px root:
spacing-1 = 4px = 0.25rem
spacing-2 = 8px = 0.5rem
spacing-4 = 16px = 1rem
spacing-6 = 24px = 1.5rem
spacing-8 = 32px = 2rem
spacing-12 = 48px = 3rem
text-sm = 14px = 0.875rem
text-base = 16px = 1rem
text-lg = 18px = 1.125rem
text-xl = 20px = 1.25rem
Practical Examples
Typography system
- 1.body: font-size: 1rem (16px base)
- 2.h1: font-size: 2.25rem (36px)
- 3.small: font-size: 0.875rem (14px)
- 4.All scale with user browser preferences
Component spacing
- 1.Button padding: 0.5em 1em (scales with button font size)
- 2.Card padding: 1.5rem (fixed relative to root)
- 3.Grid gap: 1.5rem (consistent across components)
Frequently Asked Questions
What is the difference between px, rem, and em?
px (pixels) are absolute units - 16px is always 16 pixels regardless of context. rem (root em) is relative to the root font size (usually 16px on the html element). em is relative to the font size of the current element's parent. rem is preferred for responsive design as it scales with user browser settings.
How do you convert px to rem?
Divide the pixel value by the root font size: rem = px / root font size. With the browser default of 16px: 16px = 1rem, 24px = 1.5rem, 32px = 2rem. If you change the root font size to 10px (a common technique: html { font-size: 62.5%; }), then 16px = 1.6rem.
What is the difference between rem and em?
rem (root em) is always relative to the element's font size - it doesn't change based on nesting. em is relative to the font size of the element's parent, so it compounds with nested elements. Generally, rem is more predictable and preferred for font sizes, while em is useful for things that should scale with their parent context (like padding on buttons).
Why should I use rem instead of px?
rem values respect the user's browser font size preferences. Users who set their browser to a larger font size (for accessibility) will see rem-based layouts scale correctly, while px-based layouts won't scale. This makes rem critical for accessible, responsive web design.
What is the 62.5% root font size trick?
Setting html { font-size: 62.5%; } changes the root from 16px to 10px (62.5% of 16). This makes rem arithmetic easy: 1rem = 10px, 1.6rem = 16px, 2.4rem = 24px. However, this approach can cause issues with third-party components that assume 16px root, so it's debated in the community.
What base font size should I use?
Leave the browser default at 16px unless you have a strong reason to change it. The browser default is set by the user and should be respected for accessibility. Use rem values in your CSS that assume 16px root, and let users with different font size preferences benefit from automatic scaling.
When should I use em instead of rem?
Use em for properties that should scale relative to the component's own font size: padding, margin, line-height, border-radius on buttons. This way, if you change a component's font size, all its spacing scales proportionally. Use rem for font sizes themselves to avoid compounding issues in nested components.
What are common px to rem conversions?
With 16px root: 12px=0.75rem, 14px=0.875rem, 16px=1rem, 18px=1.125rem, 20px=1.25rem, 24px=1.5rem, 32px=2rem, 48px=3rem, 64px=4rem. These correspond to common font sizes in design systems like Tailwind CSS.