CSS Units Calculator

Frequently Asked Questions

When should I use em instead of rem?

Use em when you want a size relative to the current element's font size - for padding inside a button or an icon matching surrounding text. Use rem for consistent sizing relative to the page root regardless of nesting.

Why does 1px not equal 1 physical pixel?

CSS pixels are device-independent units. On a high-DPI display, 1 CSS pixel maps to 2 or more physical pixels, making layouts look crisp on HiDPI screens without requiring CSS changes.

What about vw, vh, %, and ch units?

These units depend on viewport dimensions, container size, or content that changes dynamically, so they cannot be converted to a fixed pixel value. This calculator covers the four fixed-ratio units only.

Is 1pt always 1.333px?

Yes, in CSS: 1 inch = 72 points and 1 inch = 96 CSS pixels, so 1pt = 96/72 = 1.333px by the CSS spec, independent of screen DPI.

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How It Works

CSS supports several length units for sizing text, spacing, and layout. The four most common fixed-ratio units are px (CSS pixels, device-independent), rem (relative to the root <html> font size), em (relative to the current element's own font size, which it inherits from its parent), and pt (points, a print unit defined as 1/72 inch, equal to 1.333px in CSS).

This calculator routes all conversions through pixels as a common currency. Set the root font size (default 16px, the browser default) and the parent font size for em math, enter a value in any unit, and the tool outputs the result in your target unit plus a table showing the equivalent in all four units simultaneously.

Use Cases

CSS unit conversion comes up constantly in frontend work:

Translating a designer's pixel values from a Figma or Sketch spec into rem values for an accessible stylesheet

Checking what a rem-based spacing scale looks like in pixels when reviewing a component in a browser with a non-default font size

Converting print design point sizes to CSS pixels when porting a print layout to web

Debugging unexpected sizes caused by nested em values compounding through the DOM tree

Documenting the pixel equivalent of responsive type scale values in a design system

Tips

Prefer rem for font sizes and most spacing - it scales with the user's browser font-size preference, respecting accessibility settings. Reserve px for hairline borders and details that should never scale.

The 62.5% trick sets html { font-size: 62.5% } to make 1rem = 10px, simplifying math. This changes every rem conversion in the document - use the root font size input here to account for it.

em compounds through nesting: 1.2em inside 1.2em inside 1.2em is 1.728x the root, which makes it hard to predict in deep DOM trees. Use rem for consistent spacing throughout the page.

Browser default root font size is 16px, but users can change it. Design for scalability - avoid overriding the root font size in absolute pixels.

Media query breakpoints in em (not px) respond to the user's font-size preference, making them more accessible than pixel breakpoints.

FAQ

When should I use em instead of rem?

Use em when you want a size relative to the current element's font size - for example, padding inside a button that should scale proportionally with the button's text, or an icon that should match the surrounding text size. Use rem when you want a consistent size relative to the page root regardless of nesting level.

Why does 1px not equal 1 physical pixel?

CSS pixels are device-independent units. On a high-DPI (Retina) display, 1 CSS pixel maps to 2 or more physical pixels at the device's pixel ratio. This makes layouts look crisp on HiDPI screens without requiring CSS changes.

What about vw, vh, %, and ch units?

These units depend on viewport dimensions, container size, or content metrics that change dynamically, so they cannot be converted to a fixed pixel value without knowing those measurements. This calculator covers the four fixed-ratio units only.

Is 1pt always 1.333px?

Yes, in CSS: 1 inch = 72 points and 1 inch = 96 CSS pixels, so 1pt = 96/72 = 1.333px by definition. This is a CSS-spec ratio, independent of screen DPI.

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Frequently Asked Questions

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