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Units (CSS)

An overview of the most important CSS units and some advice on when to use which

Types of units

  • Absolute: px (pixels) etc.
    • Typical example: px (pixels)
    • Drawback when using px for font size: ignores any non-standard default font size that user has configured in browser
  • Relative to parent element: %
  • Relative to viewport: vh, vw (% of viewport height, % of viewport width)
    • See also Box model
    • Also vmin, vmax (% of smallest viewport dimension, % of largest viewport dimension)
  • Relative to font size
    • rem: font size of root (HTML) element (this is set by the browser by default)
    • em: font size of parent element

When to use what

Good practice (but not necessarily best in all cases):

  • Font size on root element
    • Nothing (let your browser specify it)
    • % (percentage of default font size, still increases proportionally with font size configured in browser)
      • Most browsers set default font size to 16px by default -> setting 62.5% as font size on the root element will make 1 rem equal to 10px for default browser settings, which can make calculations easier
  • Font size on non-root element:
    • rem (typically the safe choice)
    • em (useful when font sizes need to gradually decrease when elements are nested)
  • Padding and margins: rem
  • Borders: px (visually, it often doesn't make sense for border thickness to scale with font size)
  • Width and height: % or vh
  • Top, bottom, left, right: %