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.
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.

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
andheight
. -
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:

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:

.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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