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#box-sizing

Defines how the width and height of the element are calculated: whether they include the padding and borders or not.

default box-sizing: content-box;

The width and height of the element only apply to the content of the element.

For example, this element has

  • border-width: 12px
  • padding: 30px
  • width: 200px

The full width is 24px + 60px + 200px = 284px.

The content has the defined width. The box accomodates for those dimensions.

box-sizing: border-box;

The width and height of the element apply to all parts of the element: the content, the padding and the borders.

For example, this element has

  • border-width: 12px
  • padding: 30px
  • width: 200px

The full width is 200px, no matter what.

The box has the defined width. The content accomodates for those dimensions, and ends up being 200px - 60px - 24px = 116px.