CSS

The CSS Box Model defines how elements are displayed and sized on a webpage. It consists of four layers:

1. Content – The actual text or image inside the element.
2. Padding – Space between content and border.
3. Border – Surrounds the padding and content.
4. Margin – Space between this element and other elements.

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1️⃣ Understanding the CSS Box Model

Every HTML element is treated as a box in CSS. The total size of an element is calculated as:

Total Width  = Width + Padding (left + right) + Border (left + right) + Margin (left + right)
Total Height = Height + Padding (top + bottom) + Border (top + bottom) + Margin (top + bottom)

Visual Representation

+--------------------------+  ← Margin
|                          |
|  +--------------------+  |  ← Border
|  |                    |  |
|  |   +------------+   |  |  ← Padding
|  |   |  Content   |   |  |
|  |   +------------+   |  |
|  |                    |  |
|  +--------------------+  |
|                          |
+--------------------------+

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2️⃣ CSS Box Model Properties

Property	Description
width / height	Defines the content width & height
padding	Creates space inside the border
border	Defines the outline of the box
margin	Creates space outside the box

Example:

.box {
    width: 200px;
    height: 100px;
    padding: 10px;
    border: 5px solid black;
    margin: 20px;
}

📌 Total width = 200px + (10px * 2) + (5px * 2) + (20px * 2) = 270px
📌 Total height = 100px + (10px * 2) + (5px * 2) + (20px * 2) = 170px

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3️⃣ box-sizing Property

By default, width and height only apply to the content, not padding or border. To make sure the total size includes padding and border, use:

.box {
    box-sizing: border-box;
}

Comparison:

Property	Effect
box-sizing: content-box;	Default behavior – padding & border are added outside width & height
box-sizing: border-box;	Width & height include padding & border (recommended)

✅ Best practice: Always use box-sizing: border-box; for consistent sizing.

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4️⃣ Padding in Detail

padding defines the space inside the border.

.box {
    padding: 20px; /* Applies to all sides */
}

📌 You can set different values for each side:

.box {
    padding: 10px 15px 20px 25px; /* top right bottom left */
}

or

.box {
    padding-top: 10px;
    padding-right: 15px;
    padding-bottom: 20px;
    padding-left: 25px;
}

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5️⃣ Border in Detail

The border property defines the outline of the element.

Example:

.box {
    border: 5px solid red;
}

Border Type	Example
solid	border: 2px solid black;
dashed	border: 2px dashed red;
dotted	border: 2px dotted blue;
double	border: 4px double green;
none	border: none;

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6️⃣ Margin in Detail

margin defines external spacing between elements.

.box {
    margin: 20px; /* Applies to all sides */
}

📌 You can also set individual sides:

.box {
    margin: 10px 15px 20px 25px; /* top right bottom left */
}

or

.box {
    margin-top: 10px;
    margin-right: 15px;
    margin-bottom: 20px;
    margin-left: 25px;
}

🔹 Auto Margin (Centering Elements)

.box {
    width: 300px;
    margin: 0 auto;
}

📌 This centers the element horizontally.

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7️⃣ Collapsing Margins

When two vertical margins meet, the larger one takes effect instead of adding up.

.box1 {
    margin-bottom: 50px;
}
.box2 {
    margin-top: 30px;
}

📌 Instead of 50px + 30px = 80px, the margin will be 50px (the larger one).

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8️⃣ Overflow Handling

If content exceeds the box size, use overflow:

Property	Effect
visible	Content overflows (default)
hidden	Extra content is hidden
scroll	Adds scrollbars
auto	Adds scrollbars only when needed

Example:

.box {
    width: 100px;
    height: 50px;
    overflow: hidden;
}

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9️⃣ Summary Cheat Sheet

Property	Effect
width & height	Defines element’s size
padding	Space inside the element
border	Outline of the box
margin	Space outside the element
box-sizing: border-box;	Ensures padding & border are included in width
margin: auto;	Centers the element horizontally
overflow: hidden;	Hides overflowing content

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🎯 Conclusion

✅ CSS Box Model defines how elements are sized and spaced
✅ box-sizing: border-box; simplifies layout calculations
✅ margin, padding, and border affect spacing and size
✅ Understanding these avoids layout issues