What is the difference between `width: auto` and `width: 100%`?
Jul 21, 2025 am 01:27 AMwidth:auto automatically adjusts the width according to the content, and width:100% force element width equals the width of the parent container content area; 1. width:auto will be affected by content, padding, margin, and border, and the actual width = content width padding border (excluding margin); 2. width:100% will exceed the parent container, causing layout problems, because padding and border will still be superimposed; 3. In flex or table layout, auto adapts to the content, and forcefully fills the available space; 4. In responsive design, auto is more flexible, 100% suitable for filling the container, but needs to be combined with box-sizing:border-box to avoid overflow.
width: auto
and width: 100%
both seem to be used to control element width, but their behavior is actually very different. Simply put:

-
width: auto
is the default width calculation method of the browser, and it will be automatically adjusted according to the content and layout. -
width: 100%
is to force the width of the element to be equal to the width of the content area of its parent container.
The following is a few common usage scenarios to explain the difference between the two.
The influence of internal content on width of elements
When you set a width: auto
of a block-level element (such as <div> ), its width will automatically expand according to the content, and will also be affected by padding, margin, and border. for example: <img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/175303244432696.jpeg" class="lazy" alt="What is the difference between `width: auto` and `width: 100%`?"><pre class='brush:php;toolbar:false;'> .box {
width: auto;
padding: 20px;
}</pre><p> The actual width of this <code>.box
= content width padding border (excluding margin).
And if you write:

.box { width: 100%; padding: 20px; }
Then the width of this box will be limited to 100% of the parent container, but padding will still be added, causing the actual width to exceed the parent container - this may cause you to encounter the problem of "breaking the layout".
Performance in table cell or flex child
In some special layouts, such as table-cell or children under flex containers, width: auto
and width: 100%
behaviors are also different.
-
width: auto
will make these children automatically resize according to the content. -
width: 100%
will force them to fill the available space.
For example, in a flex layout:
.container { display: flex; } .item { width: 100%; }
Each .item
is stretched to fill the entire container width. And if you use auto
, it will only determine the width based on its own content.
Impact on responsive design
These two values also have different uses when doing responsive web pages:
- If you want an element to automatically adapt to the layout as the screen changes, it is usually more appropriate to keep
width: auto
because it can be flexibly adjusted to the context. -
width: 100%
is more suitable for situations where you want an element to "full" the parent container, such as pictures, input boxes, etc.
But be aware of:
- Be careful when using
100%
padding or border causing overflow. - You can use
box-sizing: border-box
to avoid this situation.
Basically that's it. The two look similar, but they are quite different in actual layout. When using it, remember to take a look at the parent container, padding and layout mode, otherwise it will be easy to get stuck.
The above is the detailed content of What is the difference between `width: auto` and `width: 100%`?. For more information, please follow other related articles on the PHP Chinese website!

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