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Know your size – CSS Units Explained: Part 2 REMs and EMs

You can see here the first part of this article: CSS Units Explained: Part 1 pixels, percentages and viewpoints units.

In CSS REM comes to the Root EM, where EM is a term borrowed from the typography world. It refers to a unit that allows setting the font-size of an element relative to the font-size of its parent 🤯.

Anyways, I promise that things will easier after we see these concepts in code.

Let's start with REM, the Root EM. How it works is that you define a font size for the body, and that font-size becomes the base unit of measure:

body { font-size: 18px; }
p {line-height: 2rem; } // 36px
h1 {margin-bottom: 3rem; } // 54px
.hero {border: 1px solid 0.5rem} // 9px

It's quite nice because if we move to a smaller screen it's enough to update just the font-size of the body all changes. All is relative to the base font-size.

On the other size, EM it's relative to the font-size of the parent container. Let's take the following code

.grid { font-size: 20px; } 
.grid .grid-cell { font-size: 1.5em; }
/* the .grid-cell will have a font-size of 20px * 1.5 = 30px 
it's the same as saying: font-size: 150%; */

But there is a small gotcha with this em. If em units are used on other properties than font-size, then the value is relative to the element’s own font-size.

So let's take again the previous code, and add a line-height prop:

.grid { font-size: 20px; } 
.grid .grid-cell { 
    font-size: 1.5em; // 30px 
    line-height: 2em;
    /* WARNING: the line-height will be 30px * 2 = 60px */

Another gotcha with em is the cascading effect. If we have this CSS:

.container { font-size: 10px; } 
.child { font-size: 2em; }

We can run into strange situations if we have some markup like this one:

<div class="container"> 
    I'm 10px; all good
    <div class="child"> 
        I'm 20px, as expected 
        <div class="child"> 
            I'm 40px, kind of big 
            <div class="child"> 
                I'm 80px, too big! 
                <!-- and getting bigger -->

There are cases were used with care em can provide us with a lot of flexibility and fluency. But, to be honest, overall I prefer REM for its predictability and consistency 🙂

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