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The top and bottom margins of blocks are sometimes combined (collapsed) into a single margin whose size is the largest of the individual margins (or just one of them, if they are equal), a behavior known as margin collapsing. Note that the margins of floating and absolutely positioned elements never collapse.

Margin collapsing occurs in three basic cases:

Adjacent siblings
The margins of adjacent siblings are collapsed (except when the latter sibling needs to be cleared past floats).
Parent and first/last child
If there is no border, padding, inline part, block formatting context created, or clearance to separate the margin-top of a block from the margin-top of its first child block; or no border, padding, inline content, height, min-height, or max-height to separate the margin-bottom of a block from the margin-bottom of its last child, then those margins collapse. The collapsed margin ends up outside the parent.
Empty blocks
If there is no border, padding, inline content, height, or min-height to separate a block's margin-top from its margin-bottom, then its top and bottom margins collapse.

Some things to note:

  • More complex margin collapsing (of more than two margins) occurs when the above cases are combined.
  • These rules apply even to margins that are zero, so the margin of a first/last child ends up outside its parent (according to the rules above) whether or not the parent's margin is zero.
  • When negative margins are involved, the size of the collapsed margin is the sum of the largest positive margin and the smallest (most negative) negative margin.
  • When all margins are negative, the size of the collapsed margin is the smallest (most negative) margin. This applies to both adjacent elements and nested elements.

Examples

HTML

<p>The bottom margin of this paragraph is collapsed …</p>
<p>… with the top margin of this paragraph, yielding a margin of <code>1.2rem</code> in between.</p>

<div>This parent element contains two paragraphs!
  <p>This paragraph has a <code>.4rem</code> margin between it and the text above.</p>
  <p>My bottom margin collapses with my parent, yielding a bottom margin of <code>2rem</code>.</p>
</div>

<p>I am <code>2rem</code> below the element above.</p>

CSS

div {
  margin: 2rem 0;
  background: lavender;
}

p {
  margin: .4rem 0 1.2rem 0;
  background: yellow;
}

Result

Specifications

Specification Status Comment
CSS Level 2 (Revision 1)
The definition of 'margin collapsing' in that specification.
Recommendation Initial definition.

See also

Document Tags and Contributors

 Last updated by: mfluehr,