CSS units are one of the most fundamental aspects to consider while creating a site layout. These units will define how your design will interact on various devices.

But when it comes to CSS units, there are many options to choose from and all options are not equal. CSS units are not limited to fonts only, so as a front-end developer, you should understand how these units affect layouts, also.

In this article, I’ll show you how different CSS units work, so you can use them to create better layouts and site designs.

Types of CSS units

CSS units can be separated in the following categories:

  • Absolute units
  • Font relative units
  • Viewport relative units

Absolute units

Some units depend on certain absolute values and are not affected by any screen size or fonts. These units display may vary depending on different screen resolutions, as they depend on DPI (dots per inch) of screens.

These units are:

  • px (pixels)
  • in (inches)
  • cm (centimeter)
  • mm (millimeter)
  • pc (picas)
  • pt (points)

Pixels

Pixels are the most commonly used and accepted unit. And it’s considered the base of measurement for many other units. It provides the most consistent result among various devices.

The box element in the following example has a height of 150px and width of 150px, and it will remain the same on all screen sizes.

.box
height:150px;
width:150px;

All other absolute units like in (inches), cm (centimeter) and mm (millimeter) are real-world measurement units with very little real-world CSS use cases.

It’s still good to understand their relationship to pixels, however, so this is how they compare:

  • 1in = 96px
  • 1cm = 37.8px
  • 1mm = 3.78px

While pc (picas) and pt (points) are directly related to inches.

The relation between all absolute units can be demonstrated like this:
layout by flywheel css units unit comparison

Rem

Apart from pixels, all other absolute units have very little to no use cases for screen CSS, but since they are real-world measurement units, they can be effectively used in print CSS. (In case you are wondering about pixels, they work fine in print CSS as well!)

Font relative units

There are some units which depend on the font size or font family of the document or its parent level elements. This includes units like:

Em

Em is a relative length unit; it depends on the font size of the parent element or the document. By default, 1em is equal to 16px if is no font-size is defined.

Em inherits size from its immediate parent’s font size. Say, if a parent element has font-size:18px, then 1em will be measured as 18px for all its child’s.

Here we have a div .post with three childs, .post-item, with a title and some text.

<div class="post">
<div class="post-item">
<h2>Title</h2>
Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s
  </div>
<div class="post-item">
<h2>Title</h2>
Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s
  </div>
<div class="post-item">
<h2>Title</h2>
Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s
  </div>
</div>

Now the parent .post has a font-size:20px and the child element .post div has its own font-size:1.2em with padding:0.75em and margin:0.75em, so the computed padding and margin will be 18px (20px*1.2em*0.75em).

html
font-size:20px;


.post
font-size:20px;


.post div
font-size:1.2em;


.post-item
background:pink;
padding:0.75em;
margin:0.75em;


.post-item h2
font-size:1.5em;
margin:0.5em 0;


.post-item p
margin:0;
font-size: 1em;

As you can see, the child elements are inheriting values from the parent.
layout by flywheel css units


The advantage of using em is, if you decide to change the font-size, padding, and margin of each element proportionately, then you just have to change the parent element font size and all other elements will adjust accordingly.

That won’t be the case with absolute units like px, where you have to adjust each element individually.

This inheritance nature can be a bit tricky to manage, however, if nested elements have their own font-size in em, too.

Like in the previous demo, if you wrap the title and paragraph inside another div in one of the child items, you’ll will see strange results.

<div class="post-item">
<div>
<h2>Title</h2>
Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s
  	</div>
</div>

Because post-item has a nested div, the base size will change from 24px to 28.8px (20px*1.2em*1.2em), and font-size and margin for h2 will change to 43.2px (20px*1.2em*1.2em*1.5em) and 14.4px (20px*1.2em*1.2em*0.5em), respectively.
layout by flywheel css units

Rem

Rem can be really helpful in these types of situations because they always refer to the root element font size, not the parent element.

If you take the previous example and change the units from em to rem for .post div, you’ll see all child elements fix themselves.

.post div
  font-size:1.2rem;

Once the unit is changed from em to rem, its base changes from parent div to root element, which has a front-size:20px.

So, the calculation will be 20px*1.2rem*value and  h2 and p both will have consistent font-size and margin, regardless of their parent.
layout by flywheel css units

Ex

Ex is relative to the x-height of the current font and it’s very rarely used. The x-height measurement is not consistent; sometimes it comes from the font itself and sometimes the browser calculates itself.

Like em and rem, it depends on font but the major difference is that ex will change when you change the font-family too, which is not the case with the other two.

.box
width:150ex;
height:150ex;

Ch

Ch is similar to ex but it doesn’t depend on the x-height; rather on the width of the zero (0) character. It also changes with font-family.

.box
width:150ch;
height:150ch;

Viewport relative units

There are a few units that depend on the viewport height and width size, such as:

  • vh (viewport height)
  • vw (viewport width)
  • vmin (viewport minimum)
  • vmax (viewport maximum)

Vh

Vh (viewport height) is measured as 1vh equal to 1% of the viewport’s height. This unit is very useful for creating full height elements. Vh reacts similarly to percentage, but doesn’t depend on the parent element height.

You can use vh anywhere but the most common use case of vh is for making full height elements. In this example, the full-height class will create a blue container which will be 100% height of the viewport.

.full-height
height:100vh;
background:blue;

Vw

Vw (viewport width) is similar to vh but it considers the width, rather than height of the viewport.  1vw is equal to 1% of the viewport width. These units can be really helpful if you want to create a responsive viewport based on typography.

In this example, font-siz of h1 is 6% of the viewport width, so if viewport width is 1000px, the font size will be 60px, and if the viewport is 500px, the font size will be 30px.

h1
font-size:6vw;

Vmin

Vmin is calculated on the basis of the minimum value of the viewport’s width or height, whichever is smaller. Say you have a viewport size of 1000px tall by 800px wide. 1vmin will be equal to 8px.

Vmax

Vmax is calculated exactly the opposite of vmin, where the maximum value is considered of the viewport.

Like the previous  viewport example with 1000px tall by 800px wide, 1vmax will be equal to 10px.

Now let’s explore how you can use vmin and vmax effectively. Using our previous example, let’s say we want to have a fluid padding and margin based on the viewport size now, rather than a fixed value.

In this code snippet, I’ve set the padding to 3vmax and margin to 1.5vmin.

.post-item
background pink;
padding:3vmax;
margin:1.5vmin;

The padding and margin will change according to the viewport size.
layout by flywheel css units

Percentage(%) unit

Percentage(%) unit doesn’t belong to a any particular category mentioned above, but can be categorized as a relative unit. It is relative to its parent element.

Percentage is primarily associated with height and width of an element, but can be used anywhere where CSS length units are allowed.

Percentage is one of the most useful units for creating a responsive or fluid layout. Popular frameworks like Bootstrap, foundation, and Bulma use percentage for their base layout.

Here the full-width class will be of 100% width of its parent element.

.full-width
width: 100%;

Conclusion

I’ve covered almost all CSS units available for use. Depending on your goals, some will be better suited than others. “As a front-end developer, it’s always good to know your options because you never know which one you might need in your next project. ”

So, which units do you use most frequently to get the desired result? Let me know in the comments section below!