Mastering margin collapsing

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).

No content separating parent and descendants

The vertical margins between a parent block and its descendants can collapse. This happens when there is no separating content between them. Specifically, this occurs in two main cases:

• The margin-top of a parent collapses with the margin-top of its first in-flow descendant unless the parent has a border-top , padding-top , contains any inline content (such as text), or has clearance applied.
• The margin-bottom of a parent collapses with the margin-bottom of its last in-flow descendant unless the parent has a defined height or min-height, a border-bottom, or padding-bottom.

In both cases, creating a new block formatting context on the parent will also prevent its margins from collapsing with its children.

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 descendant 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.
• Collapsing margins is only relevant in the vertical direction.
• Margins don't collapse in a container with display set to flex or grid.

<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>

div {
  margin: 2rem 0;
  background: lavender;
}
p {
  margin: 0.4rem 0 1.2rem 0;
  background: yellow;
}