CSS Box Model
All HTML elements can be considered as boxes.
The CSS Box Model
In CSS, the term "box model" is used when talking about design and layout.
The CSS box model is essentially a box that wraps around every HTML element. It consists of: content, padding, borders and margins. The image below illustrates the box model:
Explanation of the different parts:
- Content - The content of the box, where text and images appear
- Padding - Clears an area around the content. The padding is transparent
- Border - A border that goes around the padding and content
- Margin - Clears an area outside the border. The margin is transparent
The box model allows us to add a border around elements, and to define space between elements.
Demonstration of the box model:
border: 15px solid green;
Width and Height of an Element
In order to set the width and height of an element correctly in all browsers, you need to know how the box model works.
Important: When you set the width and height properties of an element with CSS, you just set the width and height of the content area. To calculate the total width and height of an element, you must also include the padding and borders.
This <div> element will have a total width of 350px and a total height of 80px:
border: 5px solid gray;
Here is the calculation:
+ 20px (left padding + right padding)
+ 10px (left border + right border)
= 350px (total width)
50px (height of content area)
+ 20px (top padding + bottom padding)
+ 10px (top border + bottom border)
= 80px (total height)
The total width of an element should be calculated like this:
Total element width = width + left padding + right padding + left border + right border
The total height of an element should be calculated like this:
Total element height = height + top padding + bottom padding + top border + bottom border
Note: The margin property also affects the total space that the box will take up on the page, but the margin is not included in the actual size of the box. The box's total width and height stops at the border.