The box-sizing property

Decides whether the padding and border are part of an elements height and width.

Example

Both boxes below have the following CSS properties:

.box {
  width: 100px;
  border: 5px solid black;
  padding: 10px;
}

In addition, each box has the box-sizing property set to a different value. The resulting dimensions are as shown:

For border-box, the set width equals content width + padding + border.

For content-box, the set width is only for the content width. The padding and border dimensions are added to the width of the rendered element.

Default value: content-box.

Note: the same thing applies for the height property as well.

box-sizing reset method

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

This sets border-box eveywhere, but you can use content-box in specific places.

Source: https://css-tricks.com/box-sizing/