CSS3 Box Sizing

The CSS3 box-sizing property allows us to include the padding and border in an element's total width and height.

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Browser Support

The numbers in the table specify the first browser version that fully supports the property.
Numbers followed by -webkit- or -moz- specify the first version that worked with a prefix.

Property
box-sizing	10.0 4.0 -webkit-	8.0	29.0 2.0 -moz-	5.1 3.1 -webkit-	9.5

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Without the CSS3 box-sizing Property

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 appear bigger than you have set (because the element's border and padding are added to the element's specified width/height).
The following illustration shows two <div> elements with the same specified width and height:

This div is smaller (width is 300px and height is 100px).

This div is bigger (width is also 300px and height is 100px).

The two <div> elements above end up with different sizes in the result (because div2 has a padding specified):

Example

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

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

So, for a long time web developers have specified a smaller width value than they wanted, because they had to subtract out the padding and borders.
With CSS3, the box-sizing property solves this problem.

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With the CSS3 box-sizing Property

The CSS3 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 padding and border are included in 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;}