Question 1

What is the CSS box model?

Accepted Answer

The CSS box model is a rectangular box that wraps around every HTML element. It consists of four areas: content (the actual text/image), padding (space between content and border), border (the edge around padding), and margin (space outside the border).

Question 2

What is the difference between content-box and border-box?

Accepted Answer

In content-box (default), width/height only sets the content area size, padding and border are added on top. In border-box (box-sizing: border-box), width/height includes content, padding and border, making sizing more intuitive.

Question 3

How do I calculate the total element width?

Accepted Answer

Total width = content width + left padding + right padding + left border + right border + left margin + right margin. This tool calculates all these values for you automatically.

Question 4

Why should I use border-box?

Accepted Answer

Using box-sizing: border-box makes element sizing much more predictable — when you set width: 300px, the element stays 300px total regardless of padding or border. This is why many developers use the universal border-box reset.