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CSS3 box-sizing property

Recently I had to use the box-sizing property… and every time I needed to search on Internet for the correct usages. So, last time this happened I’ve decided to write a post about it, with the hope this help me memorize it - or at least next time I can google for myself 🙂

box-sizing, why?

Until a few years ago, when you assigned an element a ruleset like the following, you got a square with size 120 pixel.

.my-box {
  height: 100px;
  padding: 10px;
  width: 100px;

So, in case you absolutely want the padding, and a squared box of 100 pixel, the only solution was to change the css rules to something like:

.my-box {
  height: 80px;
  padding: 10px;
  width: 80px;

box-sizing to the rescue

To avoid this mess, CSS3 introduced the box-sizing property.

How does it work? Look at this demo:

See the demo by Bruno Scopelliti (@brunoscopelliti) on CodePen.

Both the rectangles have the same CSS class box, to which are applied the following rules:

.box {
  display: inline-block;
  height: 80px;
  padding: 20px;
  width: 180px;

The rectangles are different for the only fact that, at the second rectangle is applied the box-sizing CSS property.

.box.severe {
  box-sizing: border-box;

This forces the browser to render the rectangle with the specified width and height, and place the border and padding inside the box.

box-sizing property

Currently there are three possible values for the box-sizing property:

  • content-box: the default behavior; the box has the width and height specified, and eventually the border and the padding are outside the specified width and height.

  • border-box: force the browser to render border, and padding inside the specified width and height.

  • inherit: the value is inherited from the parent node.

Are you thinking at the support of the browsers?

Having Can I use… as reference, I can say that the support of the browser is not too bad after all.