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PX to REM & EM Converter – Scalable CSS Units

CSS Developer Tools converter EM PX REM
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PX to REM & EM Converter

Convert between pixels, REM, and EM units for scalable CSS typography

PX Pixels
px
Absolute pixel value
REM Root EM
rem
Relative to root font size
EM Parent EM
em
Relative to parent font size
Root Font Size (for REM)
16px
Parent Font Size (for EM)
16px
Conversion Formulas
REM = PX ÷ 16px
PX = REM × 16px
EM = PX ÷ 16px

Frequently Asked Questions

REM (Root EM) is always relative to the root element's font size (the <html> tag), which defaults to 16px in most browsers. This makes REM predictable and consistent across the entire document.

EM is relative to the parent element's font size, which can cause compounding effects when nested elements inherit different sizes. EM is useful for components that should scale proportionally to their container.

Divide the pixel value by the root font size (usually 16px). For example: 32px ÷ 16 = 2rem. If your root font size is set to 10px (using the 62.5% trick), then 32px ÷ 10 = 3.2rem.

REM units respect user font size preferences. If a user increases their browser's default font size for accessibility, all REM-based sizes scale proportionally. PX values remain fixed and don't respond to accessibility settings, which can break layouts for users with visual impairments.

All modern browsers default to a root font size of 16px. This means 1rem = 16px by default. Some developers use the 62.5% trick (html { font-size: 62.5%; }) to make 1rem = 10px for easier mental math, but this can cause issues with browser extensions and accessibility tools.

Use EM when you want an element to scale relative to its local context rather than the global root. Common use cases include: icon sizes that should match adjacent text, padding/margins that should scale with the component's own font size, and media query breakpoints. Use REM for global spacing, typography, and layout properties that need consistency.

By setting html { font-size: 62.5%; }, the root font size becomes 10px (62.5% of 16px). This makes conversions trivial: 24px = 2.4rem. However, this approach has fallen out of favor because it can interfere with browser extensions, accessibility settings, and may cause unexpected rendering in some edge cases.

Not always. While the browser default is 16px, the root font size can be changed by CSS (e.g., html { font-size: 20px; }) or by the user through browser settings. That's the power of REM — it adapts to whatever the root font size is, making your design flexible and accessible.

Yes. Since EM is relative to the parent element's computed font size, nested elements with EM values can multiply. For example, if a parent has font-size: 1.5em (24px if parent is 16px) and its child has font-size: 1.5em, the child will be 36px (24 × 1.5), not 24px. This compounding effect is why REM is often preferred for consistent typography.
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