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

Without the CSS 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 appears bigger than you have set (because the element's border and padding are added to the element's specified width/height).

EXAMPLE:

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

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

With the CSS box-sizing Property

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, padding and border are included in the width and height.

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

Without the CSS box-sizing Property

With the CSS box-sizing Property