Box sizing in CSS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the CSS property box-sizing control?

It controls how the total width and height of an element are calculated, including content, padding, and border.

beginner

What is the default value of box-sizing in CSS?

The default value is content-box, which means width and height apply only to the content area, excluding padding and border.

intermediate

Explain the difference between content-box and border-box values of box-sizing.

content-box: width and height include only content.
border-box: width and height include content, padding, and border.

intermediate

Why is box-sizing: border-box; often recommended for modern web design?

Because it makes sizing easier by including padding and border inside the set width and height, preventing unexpected element size growth.

beginner

How can you apply box-sizing: border-box; to all elements on a webpage?

Use the CSS rule:

*, *::before, *::after { box-sizing: border-box; }

This ensures consistent sizing for all elements and their pseudo-elements.

What part of an element does width control when box-sizing is set to content-box?

A Only the content area

B Content plus padding

C Content, padding, and border

D Content, padding, border, and margin

content-box means width applies only to the content area, excluding padding and border.

If an element has width 200px, padding 20px, and border 5px, what is its total width with box-sizing: border-box;?

A 150px

B 250px

C 230px

D 200px

With border-box, the total width stays 200px including padding and border.

Which CSS selector is best to apply box-sizing: border-box; globally?

A body

B *

C *, *::before, *::after

D html

Using *, *::before, *::after covers all elements and their pseudo-elements.

What happens if you don’t set box-sizing and add padding to an element?

A Element size grows beyond set width/height

B Element size shrinks

C Padding is ignored

D Element size stays the same

With default content-box, padding adds to the element size, making it bigger.

Which box-sizing value makes it easier to create layouts that don’t break when adding padding or borders?

A content-box

B border-box

C padding-box

D margin-box

border-box includes padding and border inside the width and height, simplifying layout.

Describe how the box-sizing property affects the size of an element and why it matters in web design.

Think about how padding and border affect the total size.

Explain how to set up your CSS so that all elements use box-sizing: border-box; and why this is helpful.

Remember the universal selector and pseudo-elements.

Practice

1. What does the CSS property box-sizing: border-box; do?

easy

A. Removes padding and border from the element.

B. Adds padding and border outside the element's width and height.

C. Sets the element's width and height to auto.

D. Includes padding and border inside the element's width and height.

Solution

1. Step 1: Understand box-sizing options

   The content-box model adds padding and border outside the width and height, while border-box includes them inside.

2. Step 2: Apply to border-box

   Using box-sizing: border-box; means the total size includes padding and border, making layout easier.

3. Final Answer:

   Includes padding and border inside the element's width and height. -> Option D

4. Quick Check:

   box-sizing: border-box = padding and border inside [OK]

Hint: Remember border-box keeps size fixed including padding and border [OK]

Common Mistakes:

• Confusing border-box with content-box behavior
• Thinking padding is always outside size
• Ignoring border width in calculations

2. Which of the following is the correct CSS syntax to set box sizing to include padding and border inside the element's size?

easy

A. box-sizing: border-box;

B. box-sizing: size-box;

C. box-sizing: padding-box;

D. box-sizing: content-box;

Solution

1. Step 1: Recall valid box-sizing values

   The valid values are content-box and border-box. Others like padding-box or size-box are invalid.

2. Step 2: Identify correct value for including padding and border

   border-box includes padding and border inside the element's size.

3. Final Answer:

   box-sizing: border-box; -> Option A

4. Quick Check:

   Correct syntax for including padding and border = border-box [OK]

Hint: Only content-box and border-box are valid values [OK]

Common Mistakes:

• Using invalid box-sizing values
• Mixing up content-box and border-box
• Forgetting the colon or semicolon in CSS

3. Given this CSS:

div {
  width: 200px;
  padding: 20px;
  border: 10px solid black;
  box-sizing: content-box;
}

What is the total width of the div element as rendered in the browser?

medium

A. 200px

B. 240px

C. 260px

D. 260px plus margin

Solution

1. Step 1: Calculate width with content-box

   With content-box, width is content only. Padding and border add outside width.
   Width = content width + 2 * padding + 2 * border = 200 + 40 + 20 = 260px.

2. Step 2: Confirm total width

   Total width rendered = 260px (margin not included here).

3. Final Answer:

   260px -> Option C

4. Quick Check:

   content-box width = content + padding + border = 260px [OK]

Hint: Add padding and border twice to content width for content-box [OK]

Common Mistakes:

• Forgetting to double padding and border for both sides
• Confusing content-box with border-box
• Including margin in width calculation

4. You have this CSS:

p {
  width: 150px;
  padding: 15px;
  border: 5px solid blue;
  box-sizing: border-box;
}

But the paragraph is wider than 150px on the page. What is the likely cause?

medium

A. There is extra margin or display property affecting width.

B. The browser does not support box-sizing.

C. Padding and border are added outside width with border-box.

D. Width property is ignored with border-box.

Solution

1. Step 1: Understand border-box behavior

   With border-box, padding and border are inside the width, so total width should be 150px.

2. Step 2: Identify other layout factors

   If the element is wider, likely margin, or display (like inline-block with whitespace) or parent constraints affect size.

3. Final Answer:

   There is extra margin or display property affecting width. -> Option A

4. Quick Check:

   border-box includes padding/border; extra width = margin or display [OK]

Hint: Check margin and display if border-box width seems off [OK]

Common Mistakes:

• Assuming border-box adds padding outside width
• Ignoring margin or inline-block spacing
• Thinking width is ignored with border-box

5. You want a fixed 300px wide button including padding and border. Which CSS will achieve this correctly?

hard

A. button {
width: 300px;
padding: 20px;
border: 5px solid black;
box-sizing: content-box;
}

B. button {
width: 300px;
padding: 20px;
border: 5px solid black;
box-sizing: border-box;
}

C. button {
width: 270px;
padding: 20px;
border: 5px solid black;
box-sizing: border-box;
}

D. button {
width: 300px;
padding: 0;
border: 0;
box-sizing: border-box;
}

Solution

1. Step 1: Understand fixed width with padding and border

   To keep total width 300px including padding and border, use box-sizing: border-box;.

2. Step 2: Check options

   button {
     width: 300px;
     padding: 20px;
     border: 5px solid black;
     box-sizing: content-box;
   }

   uses content-box, so total width > 300px.
   

   button {
     width: 300px;
     padding: 20px;
     border: 5px solid black;
     box-sizing: border-box;
   }

   uses border-box with 300px width, so total size is 300px.
   

   button {
     width: 270px;
     padding: 20px;
     border: 5px solid black;
     box-sizing: border-box;
   }

   sets width 270px with border-box, making total smaller than 300px.
   

   button {
     width: 300px;
     padding: 0;
     border: 0;
     box-sizing: border-box;
   }

   removes padding and border, which is not desired.

3. Final Answer:

   Option B CSS code -> Option B

4. Quick Check:

   border-box + width = total fixed size including padding/border [OK]

Hint: Use border-box to keep total width fixed including padding and border [OK]

Common Mistakes:

• Using content-box and expecting fixed total width
• Adjusting width incorrectly with border-box
• Removing padding or border instead of adjusting box-sizing