CSS Box Sizing

The CSS box-sizing property defines how to calculate the width and height of an element: should the calculation include padding and borders, or not.

By default, the width and height of an element is calculated like this:

• width + padding + border = actual width of an element

• height + padding + border = actual height of an element

This means: When you set the width/height of an element, the element often appears bigger than you have set (because the element's border and padding are added to the element's specified width/height).

The following illustrates two <div> elements with the same specified width and height. The two <div> elements end up with different sizes (because div2 has a large padding specified):

This div is has width: 300px and height: 100px.

This div also has width: 300px and height: 100px.

Example

.div1 {
  width: 300px;
  height:
100px;
  border: 1px solid blue;
}

.div2 {
  width: 300px;
  height: 100px;
  padding: 50px;
  border: 1px solid red;
}

The box-sizing property solves this problem.

The CSS box-sizing Solution

The box-sizing property allows us to include the padding and border in an element's total width and height.

If you set box-sizing: border-box; on an element, the padding and border are included in the calculation of the width and height:

Both divs are the same size now!

Hooray!

Here is the same example as above, with box-sizing: border-box; added to both <div> elements:

Example

.div1 {
  width: 300px;
  height:
100px;
  border: 1px solid blue;
  box-sizing: border-box;
}

.div2 {
  width: 300px;

height: 100px;
  padding: 50px;
  border: 1px solid red;
  box-sizing: border-box;
}

Since the result of using the box-sizing: border-box; is so much better, many developers want all elements on their pages to work this way.

The code below ensures that all elements are sized in this more intuitive way. Many browsers already use box-sizing: border-box; for many form elements (but not all - which is why inputs and text areas look different at width: 100%;).

Applying this to all elements is safe and wise:

Example

* {
  box-sizing: border-box;
}

CSS Box Sizing Property