width

The width CSS property specifies the width of an element. By default, the property defines the width of the content area. If box-sizing is set to border-box, however, it instead determines the width of the border area.

/* <length> values */
width: 300px;
width: 25em;

/* <percentage> value */
width: 75%;

/* Keyword values */
width: 25em border-box;
width: 75% content-box;
width: max-content;
width: min-content;
width: available;
width: fit-content;
width: auto;

/* Global values */
width: inherit;
width: initial;
width: unset;

The width property is specified as either:

• one of the following keyword values: available, min-content, max-content, fit-content, auto.
• a <length> or a <percentage>. This may optionally be followed by one of the following keywords: border-box, content-box.

<length> (en-US)

Defines the width as an absolute value.

<percentage> (en-US)

Defines the width as a percentage of the containing block's width. If the width of the containing block depends on the width of the element, the resulting layout is undefined.

border-box

If present, the preceding <length> (en-US) or <percentage> (en-US) is applied to the element's border box.

content-box

If present, the preceding <length> (en-US) or <percentage> (en-US) is applied to the element's content box.

auto

The browser will calculate and select a width for the specified element.

fill

Use the fill-available inline size or fill-available block size, as appropriate to the writing mode.

max-content

The intrinsic preferred width.

min-content

The intrinsic minimum width.

available

The containing block width minus horizontal margin, border and padding.

fit-content

The larger of:

• the intrinsic minimum width
• the smaller of the intrinsic preferred width and the available width

auto | (en-US) <length> (en-US) | (en-US) <percentage> (en-US) | (en-US) min-content | (en-US) max-content | (en-US) fit-content | (en-US) fit-content(<length-percentage>)
where 
<length-percentage> = <length> (en-US) | (en-US) <percentage> (en-US)

p.goldie {
  background: gold;
}

<p class="goldie">The Mozilla community produces a lot of great software.</p>

.px_length {
  width: 200px;
  background-color: red;
  color: white;
  border: 1px solid black;
}

.em_length {
  width: 20em;
  background-color: white;
  color: red;
  border: 1px solid black;
}

<div class="px_length">Width measured in px</div>
<div class="em_length">Width measured in em</div>

.percent {
  width: 20%;
  background-color: silver;
  border: 1px solid red;
}

<div class="percent">Width in percentage</div>

p.maxgreen {
  background: lightgreen;
  width: intrinsic;           /* Safari/WebKit uses a non-standard name */
  width: -moz-max-content;    /* Firefox/Gecko */
  width: -webkit-max-content; /* Chrome */
}

<p class="maxgreen">The Mozilla community produces a lot of great software.</p>

p.minblue {
  background: lightblue;
  width: -moz-min-content;    /* Firefox */
  width: -webkit-min-content; /* Chrome */
}

<p class="minblue">The Mozilla community produces a lot of great software.</p>

• box model, height, box-sizing, min-width (en-US), max-width (en-US)