Percentage Height in HTML 5/CSS [Answered]

Query explained:

I am trying to set a <div> to a certain percentage height in CSS, but it just remains the same size as the content inside it. When I remove the HTML 5 <!DOCTYTPE html> however, it works, the <div> taking up the whole page as desired. I want the page to validate, so what should I do?

I have this CSS on the <div>, which has an ID of page:

#page {
    padding: 10px;
    background-color: white;
    height: 90% !important;
}

How to use percentage height in HTML5 or CSS? Answer #1:

I am trying to set a div to a certain percentage height in CSS

Percentage of what?

To set a percentage height, its parent element(*) must have an explicit height. This is fairly self-evident, in that if you leave height as auto, the block will take the height of its content… but if the content itself has a height expressed in terms of percentage of the parent you’ve made yourself a little Catch 22. The browser gives up and just uses the content height.

So the parent of the div must have an explicit height property. Whilst that height can also be a percentage if you want, that just moves the problem up to the next level.

If you want to make the div height a percentage of the viewport height, every ancestor of the div, including <html> and <body>, have to have height: 100%, so there is a chain of explicit percentage heights down to the div.

(*: or, if the div is positioned, the ‘containing block’, which is the nearest ancestor to also be positioned.)

Alternatively, all modern browsers and IE>=9 support new CSS units relative to viewport height (vh) and viewport width (vw):

div {
    height:100vh; 
}

Answer #2:

You need to set the height on the <html> and <body> elements as well; otherwise, they will only be large enough to fit the content. For example:

<!DOCTYPE html>
<title>Example of 100% width and height</title>
<style>
html, body { height: 100%; margin: 0; }
div { height: 100%; width: 100%; background: red; }
</style>
<div></div>

Answer #3:

The first answer will let you know in which cases “height: XX%;” will or won’t work.

If you want to create an element with a set ratio (height: % of its own width), the best way to do that is by effectively setting the height using padding-bottom. Example for square:

<div class="square-container">
  <div class="square-content">
    <!-- put your content in here -->
  </div>
</div>

.square-container {  /* any display: block; element */
  position: relative;
  height: 0;
  padding-bottom: 100%; /* of parent width */
}
.square-content {
  position: absolute;
  left: 0;
  top: 0;
  height: 100%;
  width: 100%;
}

Answer #4:

You can use 100vw / 100vh. CSS3 gives us viewport-relative units. 100vw means 100% of the viewport width. 100vh; 100% of the height.

    <div style="display:flex; justify-content: space-between;background-color: lightyellow; width:100%; height:85vh">
        <div style="width:70%; height: 100%; border: 2px dashed red"></div>
        <div style="width:30%; height: 100%; border: 2px dashed red"></div>
    </div>

Answer #5:

In order to use percentage(%), you must define the % of its parent element. If you use body{height: 100%} it will not work because its parent have no percentage in height. In that case in order to work that body height you must add this in html{height:100%}

In other cases to get rid of that defining parent percentage you can use

body{height:100vh}

vh stands for viewport height.

Answer #6:

Sometimes, you may want to conditionally set the height of a div, such as when the entire content is less than the height of the screen. Setting all parent elements to 100% will cut off content when it is longer than the screen size.

So, the way to get around this is to set the min-height:

Continue to let the parent elements automatically adjust their height Then in your main div, subtract the pixel sizes of the header and footer div from 100vh (viewport units). In css, something like:

min-height: calc(100vh – 246px);

100vh is full length of the screen, minus the surrounding divs. By setting min-height and not height, content longer than screen will continue to flow, instead of getting cut off.

Hope you learned something from this post.

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