CSS Box Model

Education is not limited to just classrooms. It can be gained anytime, anywhere... - Ravi Ranjan (M.Tech-NIT)

The CSS Box Model

All HTML elements can be considered as boxes. 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: margins, borders, padding, and the actual content. 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. 

Example

div {
    width: 300px;
    border: 25px solid green;
    padding: 25px;
    margin: 25px;

}

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.

 

Here is the math:

320px (width)
+ 20px (left + right padding)
+ 10px (left + right border)
+ 0px (left + right margin)
= 350px

The total width of an element should be calculated like this:

Total element width = width + left padding + right padding + left border + right border + left margin + right margin

The total height of an element should be calculated like this:

Total element height = height + top padding + bottom padding + top border + bottom border + top margin + bottom margin