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Table of Contents
When Does Margin Collapse Happen?
Adjacent Sibling Elements
Parent and Child Margins
It Doesn't Happen Everywhere
Home Web Front-end CSS Tutorial What is margin collapsing and how does it happen?

What is margin collapsing and how does it happen?

Jun 27, 2025 am 01:18 AM

<p>Margin folding in CSS refers to the phenomenon that the vertical margins of adjacent elements are merged into one margin, which mainly occurs in three situations: 1. Between the parent element and the first or last child element; 2. Between the adjacent brother elements; 3. Between the upper and lower margins of empty elements. For example, the upper and lower margins of two consecutive paragraphs will be combined into the larger value instead of adding them; when the parent element has no border or inner margin, the outer margin of the child element will affect the parent element's position; you can prevent collapse by adding inner margins, borders, or setting overflow attributes. Margin folding only occurs in the vertical direction, does not occur in the horizontal direction, and does not occur in the case of using flex, grid layout, or in the presence of floating or absolute positioning.

<p>What is margin collapse and how does it happen?

<p> Sometimes when you're laying out a webpage using CSS, the vertical margins of elements might merge into one margin instead of staying separate. This behavior is called margin collapse , and while it's part of how CSS works by default, it can be confusing if you're not expecting it.

When Does Margin Collapse Happen?

<p> Margin collapse typically occurs in three main situations:

  • Between parent and first/last child elements
  • Between adjacent siblings
  • On empty elements (where top and bottom margins have nothing separating them)
<p> It only happens for vertical margins — horizontal margins (left and right) don't collapse.

Adjacent Sibling Elements

<p> This is probably the most common case you'll run into. Imagine two paragraphs sitting one after another:

 <p>First paragraph</p>
<p>Second paragraph</p>
<p> If both have a margin-bottom and margin-top , their margins won't just add up — they'll collapse into a single margin equal to the larger of the two.

<p> For example:

  • First <p> has margin-bottom: 20px
  • Second <p> has margin-top: 30px
<p> Instead of having 50px of space between them, there will only be 30px because the margins collapsed.

<p> This helps prevent unnecessary spacing when building standard document-style layouts.

Parent and Child Margins

<p> Another tricky case is when a child element's margin touches the top or bottom edge of its parent. If the parent has no padding, border, or content to separate itself from the child's margin, the margins will collapse.

<p> For example:

 <div class="parent">
  <p>Child paragraph with margin-top: 20px</p>
</div>
<p> If the
doesn't have a border, padding, or any other content before the <p>

, then that 20px top margin on the <p>

will actually push the whole parent down — as if the parent itself had that margin.<p> To stop this from happening, you can:

  • Add padding-top to the parent
  • Use a border like border: 1px solid transparent
  • Apply overflow: hidden on the parent
<p> These tricks create a “barrier” that prevents the margins from touching and collapse.

It Doesn't Happen Everywhere

<p> There are several cases where margin collapse won't happen. For example:

  • If either element has a non-auto display value like flex , grid , or inline-block
  • If there's a border, padding, or background between the margins
  • If one of the elements is floated or absolutely positioned
<p> Also, remember again — horizontal margins never collapse , so you won't see this behavior with left or right margins.

<p> So yeah, margin collapse is just how CSS handles spacing by default in certain vertical scenarios. It's not a bug — it's intentional — but it can definitely trip you up if you're not aware of how it works.

<p> Basically that's it.

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