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.

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

Demonstration of the box model:

div {
  width: 300px;
  border: 15px solid green;
  padding: 50px;
  margin: 20px;
}

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.

Example

This <div> element will have a total width of 350px and a total height of 80px:

div {
  width: 320px;
  height: 50px;
  padding: 10px;
  border: 5px solid gray;
  margin: 0;
}

Here is the calculation:

  320px (width of content area)
+ 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.