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Table of Contents
How Size is Calculated
The box-sizing Property
Home Web Front-end CSS Tutorial What is the CSS box model?

What is the CSS box model?

Jul 30, 2025 am 01:41 AM
css box model

The CSS box model determines an element's size and spacing through four layers: 1. Content – the innermost area defined by width and height; 2. Padding – transparent space inside the border; 3. Border – the line surrounding content and padding; 4. Margin – transparent space outside the border separating elements. Total width and height include all these components, calculated as margin border padding content. Using box-sizing: border-box changes this so width and height include content, padding, and border, making layout control more predictable, which is why many developers apply * { box-sizing: border-box; } globally to ensure consistent sizing across elements. Understanding the box model is essential for accurate layout design and avoiding unexpected spacing issues.

What is the CSS box model?

The CSS box model is how browsers calculate the total width, height, and spacing of an HTML element. Every element on a webpage is treated as a rectangular box, and the box model defines how that box is sized and spaced.

What is the CSS box model?

It consists of four main parts, layered from innermost to outermost:

  • Content – The actual text, image, or other media inside the element. Defined by width and height.
  • Padding – Transparent space around the content, inside the element’s border. Controlled by the padding property.
  • Border – A line that surrounds the content and padding. Set using the border property.
  • Margin – Transparent space outside the border, separating the element from neighboring elements. Controlled by margin.

How Size is Calculated

By default, the total space an element takes up on the page is:

What is the CSS box model?

Total width = margin-left border-left padding-left width padding-right border-right margin-right
Total height = margin-top border-top padding-top height padding-bottom border-bottom margin-bottom

For example, if you have:

What is the CSS box model?
.box {
  width: 200px;
  padding: 20px;
  border: 5px solid black;
  margin: 10px;
}

The total width becomes:
10 (left margin) 5 (left border) 20 (left padding) 200 (content width) 20 (right padding) 5 (right border) 10 (right margin) = 270px

The box-sizing Property

To make sizing more intuitive, use:

box-sizing: border-box;

With border-box, the width and height include the content, padding, and border — but not the margin. So if you set width: 200px with box-sizing: border-box, the total content padding border will fit within 200px, making layouts easier to manage.

Most developers use this reset to avoid surprises:

* {
  box-sizing: border-box;
}

That way, all elements use the same sizing model.

Basically, the box model is foundational — understanding it helps you control layout, spacing, and avoid unexpected size issues.

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