The Edges

The browser creates a rectangle for each element in the HTML document. The Box Model describes how the padding, border, and margin are added to the content to create this rectangle.

CSS Box Model

Diagram from CSS2.2 Working Draft

The perimeter of each of the four areas is called an edge. Each edge defines a box.

• The innermost rectangle is the content box. The width and height of this depends on the element’s rendered content (text, images and any child elements it may have).
• Next is the padding box, as defined by the padding property. If there is no padding width defined, the padding edge is equal to the content edge.
• Then we have the border box, as defined by the border property. If there is no border width defined, the border edge is equal to the padding edge.
• The outermost rectangle is the margin box, as defined by the margin property. If there is no margin width defined, the margin edge is equal to the border edge.

Example

div {
    border: 5px solid red;
    margin: 50px;
    padding: 20px;
}

This CSS styles all div elements to have a top, right, bottom and left border of 5px in width; a top, right, bottom and left margin of 50px; and a top, right, bottom, and left padding of 20px. Ignoring content, our generated box will look like this:

Box Model for Above Example

Screenshot of Google Chrome’s Element Styles panel

• As there is no content, the content region (the blue box in the middle) has no height or width (0px by 0px).
• The padding box by default is the same size as the content box, plus the 20px width on all four edges we’re defining above with the padding property (40px by 40px).
• The border box is the same size as the padding box, plus the 5px width we’re defining above with the border property (50px by 50px).