CSS

How the CSS Box Model Works

The box model is a term thrown around in CSS with very little context, but is probably the most important and fundamental thing you can know in CSS. Simply put, the box model determines, the size, margin, and padding of any object on the page. It also refers to the weird way CSS handles 'inline' and 'block' content.

The Box Model

In HTML, every element creates a box. Some of these elements, such as span elements are inline, meaning they are in line with text, rather than big block elements on a page (think, navigation, or headers).

Other elements, like div are 'block' elements. Blocks have a width and height which can span the entire page by default, while inline elements only act that way if the text or content within them stretches that far.

Regardless of whether an element is inline or a block, all elements have a number of attributes, such as width, height, padding within an element, and the margin or gap outside of it. The diagram below illustrates all the attributes which an HTML element can have:

• margin is the space outside (around) an HTML element.
• border is the line around the element, enclosing both padding and the width/height.
• padding is the space around the text and the edge of the element.
• width and height only refer to the space within that, excluding the padding.

Try out the CSS box model

To help you understand how these attributes affect an HTML element, below I've created is a simple div with interactive sliders. You can use the sliders to adjust the box model properties, and see how it affects the div:

Box Model Properties

We have 5 main properties, all of which can be defined separately. Below is a div with all the box model properties applied to it:

div {
    width: 100px;
    height: 100px;
    padding: 10px;
    border: 2px solid black;
    margin: 5px;
}

For padding and margin, we can also refer to each side separately on the same line. In CSS, when we are referring to each side, the order is top, right, bottom, left. Take a look at the example below:

div {
    /*  top side padding: 10px
        right side padding: 20px
        bottom side padding: 5px
        left side padding: 10px
    */
    padding: 10px 20px 5px 10px;
}

We can also directly call out these by using the properties padding-top, padding-right, padding-bottom and padding-left. The exact same properties exist for margin, i.e:

div {
    margin-left: 20px;
}

Quick Case Study

Let's think a little bit more about how box models work. We create a new div, and give it a width of 40px, a padding of 20px, and a border of 2px, as shown below. We also add 4px of margin.

div {
    width: 40px;
    padding: 20px;
    border: 2px solid black;
    margin: 4px;
}

How big is the box?

Since the width is 40px, the padding is 20px, and the border is 2px, the total width rendered on the page is actually 84px!

How does that work? Well, the width as shown in the diagram, is the width inside the padding. Since we said padding is 20px, CSS adds 20px to the all sides of the box. That means 20px on the left, and 20px on the right, which is 40px on top of our width.

Finally, we have 2px of border the whole way around the div, which is 2px on the left hand side, and 2px on the right hand side. The result is 40px + 40px + 4px, or 84px.

Display

CSS also has another property called display, which among other things, can allow you to hide an item. Display affects the box model, by defining if an object is block or inline. For the purposes of the box model, let's think about a few key properties:

• none - the item is hidden.
• inline - the item is inline, i.e. inline with text, it cannot have a width or height added to it.
• block - the item is a block, i.e. it takes up the entire width and starts on a new line.
• inline-block - the item is inline with text, but it can have a width and height added to it in CSS.
• contents - the item is displayed as if its container does not exist, and is added to the container above.

How it looks in code

Let's take a look at an example. The code is shown below, and you can update it to see what different displays result in. Notice that by default, the element is inline, and the width and height are not applied.

span {
    display: inline;
    width: 100px;
    height: 110px;
    padding: 10px;
}

Box Sizing

The way CSS manages padding, width, and border separately has always been a point of contention in the CSS community. As such, a property has been created to remedy this, known as box-sizing. Box sizing lets us override this default behaviour.

Let's think about our 40px width box which ended up being 84px wide. We can set box-sizing to:

• border-box: the width includes border and padding. Our total width will now be 40px, even with the padding and border.
• content-box: the default behaviour, the width excludes border and padding. Our total width will now be 84px.

Now we have way more control and can set our widths with certainty that they will display as we expect them to on the page.

Borders

Borders are another way we can affect the box model. Borders can be defined as surrounding the entire element, or on a specific side, using border-top, border-right, border-bottom or border-left. Here is an example:

div {
    border: 1px solid red;
    border-top: 2px solid black;
}

A border property can be split out into separate lines too. 1px solid red can be written as:

div {
    border-width: 1px;
    border-color: red;
    border-style: solid;
}

Similarly, we can apply these to a single side, i.e. border-top-width, border-top-color or border-top-style for the top side. We can do this for any side.

The color accepts any color, and you can learn more about colors in the color section. the border-style property accepts the following values. The examples below have a 6px border width.

Border Radius

Finally, border radius lets us added rounded edges to our divs. Note, this does not affect the box model, so the size of the element remains the same, but it does affect its aesthetics. It accepts a unit like pixels, and the larger the unit the bigger the rounding. Here is an example:

div {
    border-radius: 20px;
}

Now we can have rounded edges, for design purposes! Now that we've covered the box model, let's dive into how we use fonts in CSS.

More Tips and Tricks for CSS

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• How to Create Animated Cards with WebGL and Three.js
• CSS Colors
• How to make a div the same height as the browser window
• The Quick Guide to Dark Mode
• Stripe-Like Smooth Transition CSS Only Menu
• Putting Javascript in Your CSS with Paint Worklets
• Creating Custom, Interactive CSS Checkboxes
• CSS Pseudo Elements
• CSS Only Masonry Layouts with Grid