PX to REM Converter
Convert between pixels (px) and rem units instantly. Set your base font size, enter a value, and get the result — plus a quick reference table for the most common pixel values.
PX vs REM: Understanding CSS Units for Responsive and Accessible Design
When writing CSS, choosing the right unit is one of the most impactful decisions a developer can make. Two of the most commonly used length units are px (pixels) and rem (root em). While both express size in a CSS stylesheet, they behave very differently — and understanding when to use each is essential for building responsive, accessible web interfaces. This converter helps you translate between the two, but the deeper value lies in understanding why the conversion matters in the first place.
What Is a Pixel (px) in CSS?
In CSS, a pixel (px) is an absolute unit. When you set an element's font size to 16px, it will always render at 16 pixels, regardless of the user's browser settings, screen resolution, or parent element size. This predictability is useful when you need precise control — borders, box shadows, fine decorative details, and icon sizes that must not shift with user preferences.
However, the rigidity of pixels becomes a drawback for text and layout sizing. When a user increases their browser's default font size in accessibility settings (a common accommodation for low vision), elements sized in px do not respond. The user's preference is silently ignored, which can create usability barriers for people with visual impairments.
What Is a REM Unit in CSS?
The rem unit stands for 'root em.' It is relative to the font size of the root element of the document — the <html> element. By default, browsers set the root font size to 16px, which means 1rem equals 16px, 1.5rem equals 24px, and 0.875rem equals 14px. If you change the root font size, all rem values across the page scale proportionally.
This relative behavior is the key advantage of rem. When a user sets their browser's minimum font size to 20px or increases the base text size for readability, every element sized in rem units scales up automatically. Your layout adapts to the user's environment rather than overriding it — which is the foundation of accessible typography.
The Conversion Formula
Converting between px and rem requires knowing the base font size — that is, the font size applied to the <html> element. The formula is straightforward: REM = PX ÷ Base Font Size. To go the other direction: PX = REM × Base Font Size.
With the browser default of 16px, common conversions are: 12px = 0.75rem, 14px = 0.875rem, 16px = 1rem, 18px = 1.125rem, 24px = 1.5rem, 32px = 2rem. If your design system sets a different root font size — for example, 10px via html { font-size: 62.5%; } — the math changes accordingly. This calculator lets you set any base font size, so you always get the accurate conversion for your project's configuration.
Why 62.5% Root Font Size Is a Common Pattern
Many developers set html { font-size: 62.5%; } to make the base font size 10px (62.5% of the default 16px). With a 10px base, the conversion becomes trivially easy: 16px = 1.6rem, 24px = 2.4rem. This pattern reduces mental arithmetic during development.
The tradeoff is that you must then set body { font-size: 1.6rem; } or similar to restore readable text, and be consistent throughout your codebase. If you adopt this pattern, enter 10 as your base font size in this converter to get the right rem values for your project. For teams that prefer keeping the default 16px base, simply leave the base at 16.
REM vs EM: Understanding the Difference
It is worth distinguishing rem from its cousin, em. The em unit is also relative — but to the font size of the parent element, not the root. This means em values compound: if a parent element has font-size: 1.2em and a child also has font-size: 1.2em, the child's effective size is 1.44× the root, not 1.2×.
This compounding behavior makes em difficult to reason about in deeply nested components. Rem avoids this problem entirely by always referencing the root, making it more predictable and easier to maintain. For most use cases — especially font sizes and global spacing — rem is the safer, more robust choice.
Accessibility and Responsive Design Benefits
The Web Content Accessibility Guidelines (WCAG) recommend using relative units like rem for text sizing to support users who rely on browser-level font scaling. Users with dyslexia, low vision, or cognitive differences often configure their systems with larger default text sizes. Rem-based designs honor those preferences; px-based designs do not.
Beyond accessibility, rem units contribute to consistent responsive design. When combined with CSS custom properties and fluid typography techniques (such as clamp()), rem-based systems scale elegantly across device sizes. A well-constructed rem-based spacing scale means that a layout that looks right on desktop will proportionally maintain its rhythm on mobile and tablet without per-breakpoint overrides for every spacing value.
Practical Recommendations
Use rem for: font sizes, line heights, margins and padding, max-width containers, and any spacing that should participate in your design's scale. These are the values where rem's proportional behavior provides the most benefit.
Use px for: border widths (1px borders should stay 1px), box shadows, outline widths, and any decorative detail where sub-pixel rendering and exact pixel values matter. Using px for these properties does not harm accessibility, since they are visual embellishments rather than content-affecting sizes.
A practical rule of thumb: if changing the property's size would affect readability or usability, use rem. If it is a decorative detail, px is fine. Following this guideline will make your codebase more accessible, more scalable, and easier to maintain as design systems evolve.
Frequently Asked Questions
What is the default base font size for rem conversion?
The browser default base font size is 16px, which means 1rem equals 16px by default. Unless your CSS explicitly changes the root element's font size with html { font-size: ... }, you should use 16 as your base. Some developers set it to 10px (using html { font-size: 62.5%; }) to simplify mental math, in which case you would use 10 as the base.
Why should I use rem instead of px for font sizes?
Rem units respect the user's browser font size preferences, which is important for accessibility. Users with visual impairments or reading difficulties often increase their browser's default font size. Elements sized in rem scale with that preference, while elements sized in px do not — effectively overriding the user's accessibility settings. For font sizes and layout spacing, rem is the more inclusive choice.
Is it ever correct to use px in CSS?
Yes. Pixel values are appropriate for properties where exact, non-scaling values are needed: border widths, box shadows, outline sizes, and fine decorative details. A 1px border should remain 1px regardless of font size preferences. Using px for these properties does not create accessibility problems. The key is to avoid px for font sizes, margins, padding, and other sizing properties that affect readability.
How do I convert rem to px without a calculator?
Multiply the rem value by the base font size. With the standard 16px base: 1.5rem × 16 = 24px. To convert px to rem, divide by the base: 24px ÷ 16 = 1.5rem. If your base is 10px (62.5% root), the math is simpler: 1.6rem × 10 = 16px. For non-standard bases, this calculator handles the conversion for any value.
What does rem stand for in CSS?
REM stands for 'root em.' The em unit is relative to the font size of the current element's parent, while rem is always relative to the font size of the root element (the <html> element). This makes rem more predictable than em because its value does not change based on nesting depth. A 1rem value is always the same size anywhere in your document, as long as the root font size does not change.
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