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CSS box model

CSS box model

sizes + margins + borders + paddings + overflows

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jon
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5 min read

Table of contents

Each element has the following basic properties, all of which have a numerical value (usually in pixels or percentage of its parent container):

  • width (left to right length)

  • height (top to bottom length)

  • padding (inner white space)

  • margin (outer white space)

  • border (boundary space that separates the inner white space and outer white space)

In addition, there exists:

  • overflow (determines whether content inside a limited container spills over its border or not), most commonly taking on the values:

    • scroll (the browser will give the container a scroll bar)

      • auto (if the content exceeds the size of the container)

    • hidden (the content will "disappear" after the end of the container)

    • visible (the content will "spill over" after the end of the container)

  • overflow-x (restricts the overflow value to the horizontal dimension)

  • overflow-y (restricts the overflow value to the vertical dimension)

Width and height

Self-explanatory, the width determines the left-right length of an element, e.g.:

div.example {
    width: 250px;
}

Also, the height determine the top-bottom length of an element, e.g.:

div.example {
    height: 100%;
}

In addition to pixels and percentages, we could also use different units!

Min and max dimensions

If we had specified the height and width, the min-height, min-width, max-height and max-width properties can override any height and width already placed on the selector:

div.example {
    height: 140px;
    width: 90px;
}

div.example {
    min-height: 50px;
    /* this will shrink the div from 140px to 100px tall */
    max-height: 100px;
    /* this will expand the div from 90px to 100px */ 
    min-width: 100px;
    /* this will make the div 90px because of the width */
    max-width: 100%; 
}

Paddings

Padding refers to the inner whitespace of a container; it literally pads the content from the container's edges!

div.example {
    padding-top: 10px;
    padding-right: 20px;
    padding-bottom: 30px;
    padding-left: 40px;
}

div.example {
    padding: 10px 20px 30px 40px;
}

In the above example, the two declaration blocks have identical properties and values; the latter functions as a shorthand version of the former ... it moves clockwise from the top!

Also, the following long-form and short-hand remain popular among developers, as top and bottom (and left and right) often have the same values:

div.example {
    padding-top: 20px;
    padding-right: 10px;
    padding-bottom: 20px;
    padding-left: 10px;
}

div.example {
    padding: 20px 10px;
}

Margins

Margin refers to the outer whitespace of a container; the container's size does include this region that separates the container from other elements!

div.example {
    margin-top: 10px;
    margin-right: 20px;
    margin-bottom: 10px;
    margin-left: 20px;
}

div.example {
    margin: 10px 20px;
}

We notice in the above that the notation of margins imitate those of the paddings; also:

div.topcontainer {
    margin-bottom: 50px;
}

div.bottomcontainer {
    margin-top: 25px;
}

...for an HTML that has:

<div class="topcontainer">...</div>
<div class="bottomcontainer">...</div>

When two margins meet, the smaller margin combines with the larger margin; in the example above, a 50px space will appear between the two <div> elements, instead of 75px!

"Auto"

Using auto for a margin in the left-right value centers it horizontally:

div.centered {
    margin: 0 auto; 
}

Borders

The border refers to the space between the margin and the padding:

div.example {
    border-width: 10px;  
    border-style: solid; 
    border-color: red;  
}

div.example {
    border: 10px solid red; /* width style color */
}

As with paddings and margins, the border property has a shorthand but its own notation!

Border width

This "border thickness" property usually has an amount in pixels (px) but can use essentially any other units of distance!

Border style

The border style refers to the border's appearance and takes one value out of a closed set:

border-style: none;
border-style: hidden;
border-style: dotted;
border-style: dashed;
border-style: solid;
border-style: double;
border-style: groove;
border-style: ridge;
border-style: inset;
border-style: outset;

/* CLOCKWISE SHORTHANDS */

/* all four sides are solid */
border-style: solid;

/* top and bottom are dotted | left and right are solid */
border-style: dotted solid;

/* top is hidden | left and right are double | bottom is dashed */
border-style: hidden double dashed;

/* top is none | right is solid | bottom is dotted | left is dashed */
border-style: none solid dotted dashed;

Border color

Self-explanatory and can take on any CSS color notation

div.example {
    border-color: red;
    border-color: #990000;
    border-color: rgb(255, 0, 0);
    border-color: hsl(0, 50%, 50%);
}

Border radius

Furthermore, the border can have rounded edges with the border-radius property:

div.example {
    border-radius: 5px;
}

A border can become a circle with a border-radius value of 50%:

div.example { 
    border-radius: 50%;
}

We can even choose which corner to set our border "radii":

div.example {
    border-top-left-radius: 10%;
    border-top-right-radius: 20%;
    border-bottom-right-radius: 30%;
    border-bottom-left-radius: 40%;
}

div.example {
    border-radius: 10% 20% 30% 40%;
}

(Of course, the latter serves as the "clockwise shorthand" for the former in the above example!)

Overflows

The overflow refers to how much of the content the container will display:

div.example {
    /* browser will show a scrollbar on the div */
    overflow: scroll; 
}

div.example {
    /* browser will show a scrollbar only when necessary */
    overflow: auto; 
}

div.example {
    /* content will 'disappear' after the end of the container */
    overflow: hidden; 
}

div.example {
    /* content will 'spill over' after the end of the container */
    overflow: visible;
}

There also exist overflow-x and overflow-y to limit the overflow value in, respectively, the horizontal and vertical directions!

  • Overflow-x will apply to the horizontal (left-right) direction

  • Overflow-y will apply to the vertical (top-bottom) direction

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