CSS – why doesn’t percentage height work?

292,056

Solution 1

The height of a block element defaults to the height of the block's content. So, given something like this:

<div id="outer">
    <div id="inner">
        <p>Where is pancakes house?</p>
    </div>
</div>

#inner will grow to be tall enough to contain the paragraph and #outer will grow to be tall enough to contain #inner.

When you specify the height or width as a percentage, that's a percentage with respect to the element's parent. In the case of width, all block elements are, unless specified otherwise, as wide as their parent all the way back up to <html>; so, the width of a block element is independent of its content and saying width: 50% yields a well defined number of pixels.

However, the height of a block element depends on its content unless you specify a specific height. So there is feedback between the parent and child where height is concerned and saying height: 50% doesn't yield a well defined value unless you break the feedback loop by giving the parent element a specific height.

Solution 2

A percentage value in a height property has a little complication, and the width and height properties actually behave differently to each other. Let me take you on a tour through the specs.

`height` property:

Let's have a look at what CSS Snapshot 2010 spec says about height:

The percentage is calculated with respect to the height of the generated box's containing block. If the height of the containing block is not specified explicitly (i.e., it depends on content height), and this element is not absolutely positioned, the value computes to 'auto'. A percentage height on the root element is relative to the initial containing block. Note: For absolutely positioned elements whose containing block is based on a block-level element, the percentage is calculated with respect to the height of the padding box of that element.

OK, let's take that apart step by step:

The percentage is calculated with respect to the height of the generated box's containing block.

What's a containing block? It's a bit complicated, but for a normal element in the default static position, it's:

the nearest block container ancestor box

or in English, its parent box. (It's well worth knowing what it would be for fixed and absolute positions as well, but I'm ignoring that to keep this answer short.)

So take these two examples:

<div   id="a"  style="width: 100px; height: 200px; background-color: orange">
  <div id="aa" style="width: 100px; height: 50%;   background-color: blue"></div>
</div>

<div   id="b"  style="width: 100px;              background-color: orange">
  <div id="bb" style="width: 100px; height: 50%; background-color: blue"></div>
</div>

In this example, the containing block of #aa is #a, and so on for #b and #bb. So far, so good.

The next sentence of the spec for height is the complication I mentioned in the introduction to this answer:

If the height of the containing block is not specified explicitly (i.e., it depends on content height), and this element is not absolutely positioned, the value computes to 'auto'.

Aha! Whether the height of the containing block has been specified explicitly matters!

50% of height:200px is 100px in the case of #aa
But 50% of height:auto is auto, which is 0px in the case of #bb since there is no content for auto to expand to

As the spec says, it also matters whether the containing block has been absolutely positioned or not, but let's move on to width.

`width` property:

So does it work the same way for width? Let's take a look at the spec:

The percentage is calculated with respect to the width of the generated box's containing block.

Take a look at these familiar examples, tweaked from the previous to vary width instead of height:

<div   id="c"  style="width: 200px; height: 100px; background-color: orange">
  <div id="cc" style="width: 50%;   height: 100px; background-color: blue"></div>
</div>

<div   id="d"  style="            height: 100px; background-color: orange">
  <div id="dd" style="width: 50%; height: 100px; background-color: blue"></div>
</div>

50% of width:200px is 100px in the case of #cc
50% of width:auto is 50% of whatever width:auto ends up being, unlike height, there is no special rule that treats this case differently.

Now, here's the tricky bit: auto means different things, depending partly on whether its been specified for width or height! For height, it just meant the height needed to fit the contents*, but for width, auto is actually more complicated. You can see from the code snippet that's in this case it ended up being the width of the viewport.

What does the spec say about the auto value for width?

The width depends on the values of other properties. See the sections below.

Wahey, that's not helpful. To save you the trouble, I've found you the relevant section to our use-case, titled "calculating widths and margins", subtitled "block-level, non-replaced elements in normal flow":

The following constraints must hold among the used values of the other properties:

'margin-left' + 'border-left-width' + 'padding-left' + 'width' + 'padding-right' + 'border-right-width' + 'margin-right' = width of containing block

OK, so width plus the relevant margin, border and padding borders must all add up to the width of the containing block (not descendents the way height works). Just one more spec sentence:

If 'width' is set to 'auto', any other 'auto' values become '0' and 'width' follows from the resulting equality.

Aha! So in this case, 50% of width:auto is 50% of the viewport. Hopefully everything finally makes sense now!

_Footnotes

_{* At least, as far it matters in this case. spec All right, everything only kind of makes sense now.}

Solution 3

I think you just need to give it a parent container... even if that container's height is defined in percentage. This seams to work just fine: JSFiddle

html, body { 
    margin: 0; 
    padding: 0; 
    width: 100%; 
    height: 100%;
}
.wrapper { 
    width: 100%; 
    height: 100%; 
}
.container { 
    width: 100%; 
    height: 50%; 
}

Solution 4

You need to give it a container with a height. width uses the viewport as the default width

Solution 5

Another option is to add style to div

<div style="position: absolute; height:somePercentage%; overflow:auto(or other overflow value)">
 //to be scrolled 
</div>

And it means that an element is positioned relative to the nearest positioned ancestor.

View more solutions

292,056

Web_Designer

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Updated on March 03, 2020

Comments

Web_Designer about 4 years
How come a percentage value for height doesn’t work but a percentage value for width does?

For example:
```
<div id="working"></div>
<div id="not-working"></div>
```
```
#working{
    width:80%;
    height:140px;
    background:orange;
}
#not-working{
    width:80%;
    height:30%;
    background:green;
}
```
The width of #working ends up being 80% of the viewport, but the height of #not-working ends up being 0.
- Admin about 13 years
  
  Not an expert, but I'd imagine it has something to do with the assumption that you're going to be scrolling up and down a web page, but not sideways...
- Michael Benjamin over 8 years
  
  Here's a simple and clear explanation of the CSS height property with percentage values: stackoverflow.com/a/31728799/3597276
nchpmn about 13 years

Correct, but also see @mu's answer for an explanation as to why.
arthur over 11 years

correct and well explained. the difference between them is that width by default is defined by the value of its parents element, whereas heigth is define by the "value" of its content. parent's value vs content's value
MilkyTech almost 10 years

so is there no way to have height that is responsive? i.e. - you have a row of colored squares created with css background color and you want the squares to resize with the screen. jjsFiddle
Joshua Coady over 9 years

@ChrisM - Depending on how you want to use it, just set equal paddings to half the width you want the box: jsfiddle.net/en9xyv30/1 If you want content in the box, it'll likely need to be absolutely positioned -- just set top, right, bottom, and left all to 0 (or the same value other than 0 if you want some padding) and the content will resize along with the box.
Joshua Coady over 9 years

You can also use the vw unit. vw is a unit that is a percent of the viewport width. So, something like height: 10vw; width: 10vw; would scale to screen width and remain square. See caniuse.com/#feat=viewport-units for browser compatibility.
Kishore Kumar almost 8 years

thanks works for me :)
Tomasz Mularczyk over 7 years

Be careful with replacing percentages with vw unit as it will make your life really hard if you decide to put the element into another container.