Calculate dynamic height with CSS - sass

Question

I have a container and an element withing it.
<div id="container">
<div class="myEm">
...
</div>
</div>
I need to assign width and hight to that element. It should be a square. I can calculate the width relative to the parent container, but how do I pass the same value to the height:
#container .myEm {
width: calc(100% - 20px);
height: ???
}

Answer 1

One way is to make the myEm resize using padding bottom (or top) to maintain its aspect ratio. This makes myEm strictly a sizing element and you'll need an element inside that will size to itself. Here's what I mean for example:
myEm becomes:
.myEm {
height: 0;
padding-bottom: calc(100% - 20px);
position: relative;
width: calc(100% - 20px);
}
Then you need an element inside with the actual content:
.myEm-inner {
background: red;
bottom: 0;
left: 0;
position: absolute;
right: 0;
top: 0;
}
Example: https://jsfiddle.net/dgmyxs9v/1/

Answer 2

You can save yourself the wrapping div by using pseudo elements.
This solution will create the same effect – but also make the div stretch to fit content – should it become too large to be contained by the square.
/* sane box-sizing */
html { box-sizing: border-box; }
*, *::before, *::after { box-sizing: inherit; }
/* box styling */
.box {
margin: 10px;
padding: 10px;
width: calc(100% - 20px);
background-color: tomato;
font-size: 10px;
float: left;
max-width: 150px;
}
/* ––––––––––––––––––––––––––––––––––––-
This is the aspect ratio part
––––––––––––––––––––––––––––––––––––- */
.box::before {
/* aspect ratio keeper – 100% = square */
padding-bottom: 100%;
float: left;
}
.box::before,
.box::after {
content: '';
display: table;
clear: both;
}
/* second box */
.box:nth-of-type(2) { background-color: olive; }
<div class="box">
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</div>
<div class="box">
<h2>Will stretch to content</h2>
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</div>