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
If there is no border, padding, inline part, block formatting context created, or clearance to separate the
margin-topof a block from the
margin-topof one or more of its descendant blocks; or no border, padding, inline content,
min-heightto separate the
margin-bottomof a block from the
margin-bottomof one or more of its descendant blocks, then those margins collapse. The collapsed margin ends up outside the parent.
- Empty blocks
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