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The CSS box model explains why an element can render wider than its declared width: by default, that width measures only the content. Padding and border add to the outside size, while margin sits beyond the border. Setting box-sizing: border-box makes the declared width include content, padding, and border.
What is the CSS box model?
CSS lays out each element as a set of rectangular areas nested from the inside outward:
- Content: the text, image, or other material inside the element.
- Padding: the inner space around the content.
- Border: the edge surrounding the padding and content.
- Margin: the outer space beyond the border, used to separate the element from others.
Padding is inside the border; margin is outside it. The MDN CSS box model guide describes these relationships and explains how the areas contribute to layout.
Why is an element wider than its CSS width?
By default, an element uses box-sizing: content-box. Its declared width sets the content width only. Horizontal padding and border thickness are then added on both sides to determine the border-box width:
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border-box width = content width + left and right padding + left and right border
For example, MDN illustrates a 350px content width with 25px padding and a 5px border on each side. The arithmetic is 350 + 25 + 25 + 5 + 5 = 410px border-box width. Any margin lies outside that 410px and is not included in it. The figures are an instructional example, not a general sizing rule. See MDN’s box model lesson.
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To calculate the full outside space occupied between other boxes, account for margin separately. In particular, adjoining vertical margins can collapse, so the visible gap is not always the sum of both margin declarations; MDN explains margin behavior in its box model guide.
How content-box and border-box sizing differ
| Model | What declared width or height measures | Padding and border | Margin |
|---|---|---|---|
content-box (default) |
Content area | Added outside the declared content dimensions | Outside the border box |
border-box |
Content, padding, and border together | Counted within the declared dimensions | Outside the border box |
border-box does not remove padding or border; it changes which dimensions the declared size includes. If padding and border use up the declared size, the content area cannot become negative: the CSS Working Group’s CSS Box Sizing Module Level 3 draft specifies that derived content dimensions are floored at zero.
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Choosing a sizing model
Use content-box when
You want a declared width or height to describe the content area, with padding and border adding to the rendered dimensions. This is the default behavior, so it applies unless another rule changes box-sizing.
Use border-box when
You want the declared width or height to cap the combined content, padding, and border dimensions. This can make it easier to size components within a layout because adding padding or a border does not enlarge that declared border-box size. Remember that margin still affects the space outside the box.
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How to inspect a box-model sizing issue
- Check the element’s computed
box-sizingvalue in your browser’s developer tools. - Identify the declared width or height and determine whether it refers to the content alone or the border box.
- Add the relevant padding and border values on both sides if the element uses
content-box. - Check margins separately when diagnosing space between elements; adjacent vertical margins may collapse.
- If the combined content, padding, and border should fit within a fixed declared size, set
box-sizing: border-boxand inspect the resulting layout.
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