The CSS Box Model

Understanding the CSS Box Model

The CSS box model is fundamental to understanding how elements are sized and spaced on a webpage. Every HTML element is essentially a rectangular box.

Box Model Components

From inside to outside:

1. Content - The actual content (text, images, etc.)
2. Padding - Space between content and border
3. Border - The border around the padding
4. Margin - Space outside the border

Box Model Properties

Padding

/* All sides */
padding: 20px;

/* Top/bottom, left/right */
padding: 10px 20px;

/* Top, right, bottom, left */
padding: 10px 15px 20px 25px;

/* Individual sides */
padding-top: 10px;
padding-right: 15px;
padding-bottom: 20px;
padding-left: 25px;

Border

/* Shorthand */
border: 2px solid #333;

/* Individual properties */
border-width: 2px;
border-style: solid;
border-color: #333;

/* Individual sides */
border-top: 1px solid red;
border-radius: 5px; /* Rounded corners */

Margin

/* Same syntax as padding */
margin: 20px;
margin: 10px 20px;
margin: 10px 15px 20px 25px;

/* Auto centering */
margin: 0 auto;

Box-Sizing Property

/* Default behavior */
box-sizing: content-box;
/* Width = content width only */

/* Modern approach */
box-sizing: border-box;
/* Width = content + padding + border */

/* Apply to all elements */
* {
    box-sizing: border-box;
}

Practical Example

<style>
.box {
    width: 300px;
    height: 200px;
    padding: 20px;
    border: 5px solid #333;
    margin: 30px;
    background-color: #f0f0f0;
    box-sizing: border-box;
}

.content-box {
    box-sizing: content-box;
    /* Total width: 300 + 40 + 10 = 350px */
}

.border-box {
    box-sizing: border-box;
    /* Total width: 300px (includes padding and border) */
}
</style>

<div class="box border-box">
    This box uses border-box sizing
</div>