Box Model

Most HTML elements can be considered boxes, or is living inside a box.

The CSS box model describes this principal - a box wraps around all HTML elements, and it consists of: margins, borders, padding, and the actual content. This model allows us to place a border around elements and space elements in relation to other elements.

With CSS properties and values, it is possible to apply specific styles to each of these elements, and change the way they behave and/or display on the page.

The image below illustrates the box model and what you should have seen in your dev tools:

box-model

From viking codeschool

But what do these different layers mean, and how are they relating to one another?

Margin

• outside of the element

• clears an area around the border

• does NOT have a background color (transparent - shows the background-color of the container the element lives inside of)

Border

• inside the element

• a border that goes around the padding and content

• it will use the color of the element unless you give it its own border-color

Padding

• inside the element

• clears an area between the content and the border

• transparent - shows the background-color of the element

Content

• inside the element

• where text and images appear

• the element's color property actually colors the content

Layers of the Box Model

Let's get go into some more detail and practice with each of these elements of The Box Model.

Margin

The margin is the space around the element. The larger the margin, the more space between our element and the elements around it. We can adjust the margin to move our HTML elements closer to or farther from each other.

Let's start with our margins. Adjusting our margins not only moves our element relative to other elements on the page but also relative to the "walls" of the HTML document.

For instance, if we take an HTML element with a specific width (such as our <div> in the editor) and set its margin to auto - this tells the document to automatically put equal left and right margins on our element, centering it on the page.

If you want to specify a particular margin, to a particular side, you can do it like this:

div {
  margin-top: 1px;
  margin-right: 2px;
  margin-bottom: 3px;
  margin-left: 4px;
}

You can also set an element's margins all at once: you just start from the top margin and go around clockwise (going from top to right to bottom to left). For instance,

div {
  margin: 1px 2px 3px 4px;
}

You can do top-bottom and side side - let's add this to our css for now:

div {
  margin: 0 auto;
}

Border

The border is the edge of the element. It's what we've been making visible every time we set the border property.

Borders can be set in two ways, just like your margins and just like we've talked about previously.

div {
  border: 5px solid black;
}

/* OR */

div {
  border-width: 5px;
  border-style: solid;
  border-color: black;
}

Padding and Content

The padding is the spacing between the content and the border. We can adjust this value with CSS to move the border closer to or farther from the content.

Padding can be set in two ways, just like your margins.

div {
  padding: 2px;
}

/* OR */

div {
  padding-top: 3px;
  padding-right: 1px;
  padding-bottom: 0px;
  padding-left: 9px;
}

/* OR */

div {
  padding: 3px 1px 0 9px;
}

Padding becomes more apparent when we have "stuff" inside the box.

Width, Height and box-sizing

So with all of these parts of an element that can have their own widths, what are we actually setting when we adjust the width and height CSS properties of an element?

It turns out that there are two ways that width and height can be interpretted and which one is used is determined by a third property called box-sizing. The CSS box-sizing prop gives you two main options:

1. content-box - this is the default behavior. When you put box-sizing: content-box on an element (or don't specify it at all), the width and height props only affect the content, meaning that padding, border, and margin are not included in these values. To calculate the full size of the element, you would add the width (of the content) to the width of the padding as well as the width of the border. Then the margin exists outside of that area.

2. border-box - this is a newer option that makes it so that your width and height props actually include content, padding, and border (all three) in their values. The only thing outside the width of the element would be its margin. Using this property setting can usually simplify layout calculations but it must be set on all elements.

Having content-box as the default may be a reason why you may have seen your HTML elements unexpectedly wrapping onto multiple lines - if you don't add the padding and border sizes into your width calculations, you can easily exceed the width of the browser and cause content to wrap to the next line. If you switch to using border-box then the only extra space outside of your element's width is the margin between elements.

Resources

• CSS Box Model on W3Schools
• CSS Box Sizing on W3Schools