Box Sizing

Marie Mosley //

The box-sizing property can make building CSS layouts easier and a lot more intuitive. It's such a boon for developers that here at CSS-Tricks we observe International Box-Sizing Awareness Day in February.

But, how is it so helpful and beloved that it deserves its own internet holiday? Time for a little bit of CSS history.

Box Model History

Since the dawn of CSS, the box model has worked like this by default:

width + padding + border = actual visible/rendered width of an element's box

height + padding + border = actual visible/rendered height of an element's box

This can be a little counter-intuitive, since the width and height you set for an element both go out the window as soon as you start adding padding and borders to the element.

Back in the old days of web design, early versions of Internet Explorer (<= IE6) handled the box model differently when it was in "quirks mode". The "quirks" box model worked like this:

width = actual visible/rendered width of an element's box

height = actual visible/rendered height of an element's box

The border and padding values were moved inside the element's box, cutting into the width/height of the box rather than expanding it.


The box at the top shows the default box model. The box at the bottom shows what was once the "quirks mode" interpretation of the box model.

Some people preferred this "quirky" interpretation of the box model and considered it more intuitive. It's a valid point. Having the actual visible width of a box turn out differently from what you declared in the CSS is a bit mind bending.

But, in the days of fixed-width design, it wasn't particularly complicated to work with the default box model once you understood it. You could do a bit of arithmetic to figure out how many pixels you needed to trim off of an element's declared width or height to accommodate its padding and border. The problem for present-day developers is that those absolute pixel lengths don't translate to responsive design, so the same math doesn't apply anymore.

As responsive design (or, as it was once known, "fluid" or "liquid" layout) started to gain popularity, developers and designers wished for an update to the box model. The great designer Jon Hicks, known for his excellent fluid width designs, had this to say on the subject in the CSS Wishlist we put together in 2008:

I would love a different box model! I find it bizarre that padding and border add the width of an object, and would love to be able to give something like a textarea 100% width and 3px padding without worrying what it’s going to do the layout. Perhaps something like padding-inside as a new selector?

In that vein I also wish I could specify a 100% width for an element, minus a set fixed width. Again, very useful when creating fluid designs with form elements!

Present-Day box-sizing

Those wishes were granted when the box-sizing property was introduced in CSS3. Though box-sizing has three possible values (content-box, padding-box, and border-box), the most popular value is border-box.

Today, the current versions of all browsers use the original "width or height + padding + border = actual width or height" box model. With box-sizing: border-box;, we can change the box model to what was once the "quirky" way, where an element's specified width and height aren't affected by padding or borders. This has proven so useful in responsive design that it's found its way into reset styles.

At this point you may be asking yourself, "Is it possible that Old IE did something right?" Plenty of people think so.

Demo

This demo shows how border-box can help make responsive layouts more manageable. The parent div's width is 50%, and it has 3 children with different widths, padding, and margins. Click the border-box button to get all the children in the right place inside the parent.

See the Pen Box Sizing Layout Demo by CSS-Tricks (@css-tricks) on CodePen.

Good, Better, and (Probably) Best box-sizing Reset Methods

The "Old" border-box Reset

The earliest box-sizing: border-box; reset looked like this:

* {
  box-sizing: border-box;
}

This works fairly well, but it leaves out pseudo elements, which can lead to some unexpected results. A revised reset that covers pseudo elements quickly emerged:

Universal Box Sizing

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

This method selected pseudo elements as well, improving the normalizing effect of border-box. But, the * selector makes it difficult for developers to use content-box or padding-box elsewhere in the CSS. Which brings us to the current frontrunner for best practice:

Universal Box Sizing with Inheritance

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

This reset gives you more flexibility than its predecessors — you can use content-box or padding-box (where supported) at will, without worrying about a universal selector overriding your CSS. We went into more depth on this technique and the reasoning behind it in "Inheriting box-sizing Probably Slightly Better Best Practice". One potential gripe with it is that box-sizing isn't normally inherited, so it's specialized behavior, not quite the same as something you'd normally put in a reset.

Vendor Prefixes

Every current browser supports box-sizing: border-box; unprefixed, so the need for vendor prefixes is fading. But, if you need to support older versions of Safari (< 5.1), Chrome (< 10), and Firefox (< 29), you should include the prefixes -webkit and -moz, like this:

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

Known Issues

box-sizing: border-box; is supported in the current versions of all major browsers. The less-commonly used padding-box is only supported in Firefox at the moment. There's more comprehensive information about browser support in our box-sizing almanac entry.

There are a few issues with older versions of Internet Explorer (8 and older). IE 8 doesn't recognize border-box on elements with min/max-width or min/max-height (this used to affect Firefox too, but it was fixed in 2012). IE 7 and below do not recognize box-sizing at all, but there's a polyfill that can help.