em vs. rem in CSS
Editor’s note: This article was last reviewed and updated on 7 November 2024.
In CSS, em and rem are scalable units of measurement that also specify values of properties:
em, or “em height,” is relative to the font size of the element’s parent. em is helpful when you want to scale an element’s size based on its parent’s sizerem, or “root em,” is relative to the root element. rem is helpful when you want to scale an element’s size based on the root size, regardless of the parent’s sizeWithin CSS, em and rem are both scalable units that also specify values of properties. em and rem meet web accessibility standards, and, unlike pixels (px), scale better. Consequently, they are more suited for responsive design. In this article, we dive deeper into the em and rem relative length units, demonstrating how they work, and exploring their coding patterns and differences. Finally, we’ll cover what problems they solve and when to use each.
Let’s get started!
CSS syntax involves assigning values to properties of HTML elements. The property-value pair is referred to as a CSS declaration.
Consider the code below:
h1 { color: black; background-color: rgb(190,43,111,0.5); font-weight: 800; font-size: 1.5rem; padding: 4em; margin: 10px; weight: 100%; height: 100vh; }
This code shows a CSS declaration that styles the h1 element of a webpage by assigning specific values to some properties of the element. We see that some properties, such as font-size, padding, and margin, are assigned numerical values.
In CSS, there are different numerical value types:
800 assigned to the font-weight property abovergb() function expression above. The alpha value is 0.5, referring to 50 percent opacity100% assigned to the weight property1.5rem, 10px, or 100vh. Dimensions are subdivided into length, angle, time, and resolutions. In our examples above, each of the dimensions (em, px, rem, and vh) fall under the length categoryLength values are CSS data types assigned to CSS properties, such as weight, height, margin, padding, and font-size. Length values are either absolute or relative:
px. They are not relative or dependent on anythingem, rem, and vhAs web development evolves with a growing number of devices, scalable units are preferred over fixed units because they offer the required flexibility for building responsive websites.
Now, let’s dive deeper into em and rem.
em and rem and why use them?em is a CSS unit relative to the font size of the parent element, while rem is a CSS unit relative to the font size of an html element. Both of these are scalable units, meaning they give us the ability to scale elements up and down, relative to a set value. This adds more flexibility to our designs and makes our sites more responsive.
A key reason to use scalable units like em and rem is accessibility. Accessibility enables all users, particularly those with disabilities, to successfully interact with a website. Using fixed units like px to set the values of elements, fonts, and space sizes doesn’t give us this accessibility because fixed units don’t scale.
By using scalable units like em and rem, we enable users to control the scale of the sites.
em vs. remem and rem are similar because they are both scalable units. Their values are always relative to the value of something else.
The most notable difference between em and rem is the way the browser converts them to px.
As mentioned before, em values are relative to the font-size of the nearest parent element, while rem values are relative to the root font-size, or the font-size of the html element. And when the root font-size is not explicitly set, rem values are relative to the browser’s default font-size of 16px.
This means that when the root font-size is 16px, a value of 1rem would be 16px * 1 = 16px. And a value of 10rem would be 16px * 10 = 160px.
From the above, we can see that rem values are simple and predictable, and, as a result, we can control how elements scale across the entire page easily from a single source. You can see this demonstrated below:
/* Root font-size on the document level */ html { font-size: 20px; } @media (max-width: 900px) { html { font-size: 16px; } } @media (max-width: 400px) { html { font-size: 12px; } } /* Type will scale with document */ h1 { font-size: 2.6rem; } h2 { font-size: 1.6rem; } h3 { font-size: 1.1rem; }
In the code above, a different root font-size is set for a different @media query, and the type font-size uses rem units. The result of this is that the type will scale relative to the root font-size set for each @media query.
Note that it is often considered bad practice to explicitly set the root font-size to a px value. This is because it overrides the user’s browser setting. A recommended method is to use a % unit or avoid setting the root font-size explicitly. This sets the font-size to 100% of the browser’s default font-size, which is 16px for most browsers.
Although rem units are simple and predictable, sometimes they may not give the desired control over how specific areas scale on the webpage. This is because it’s difficult for all the modules in a webpage to accurately scale up and down relative to a single value.
But because em is dependent on the font-size of the nearest parent, it gives more detailed control over how specific areas of the webpage scale. Thus, with em, we can control how the webpage scales on the modular level.
Learn more →px vs. rem vs. emAs discussed above, pixels (px) are absolute units, meaning they are fixed units that don’t change based on another element. They might not be the best choice for responsivity but they are useful for maintaining consistent sizing across elements.
em and remWhen using rem, it’s sometimes difficult for all the modules to scale up and down accurately. A proposed alternative is to use em, because components within a module are more likely to accurately scale up and down relative to the parent component. Thus, all the sidebar components would scale relative to the parent sidebar element, all the header components would scale relative to the parent header element, and so on.
em gives us control of the scale of our webpage on a modular level, but this comes with its own problems.
em valuesThe main problem experienced when working with em units is due to the effect on the inheritance of em values. Because every element inherits the font-size of its nearest parent, em values compound linearly as the level of nesting increases.
To elaborate on this, let’s build a simple app. Create a project folder and from the folder, run the following command:
npm init -y
Then install the live-server package by running:
npm i live-server
Now, replace the scripts property in the package.json file with:
"scripts": { "start": "live-server" },
After this, create an index.html file and add the following code:
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <meta http-equiv="X-UA-Compatible" content="IE=edge"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <style> .container { display: flex; justify-content: center; align-items: center; height: 100vh; width: 100wh; } .parent { font-size: 15px; } .em-child { font-size: 2em; } .rem-child { font-size: 2rem; } </style> <title>Document</title> </head> <body> <article class="container"> <div class="parent"> I'm 15px <div class="em-child"> I'm 15px * 2 = 30px <div class="em-child"> I'm 30px * 2 = 60px <div class="em-child"> I'm 60px * 2 = 120px. </div> </div> </div> </div> </article> </body> </html>
Now, run npm start to start the dev server, and we get the following:
In the example above, we demonstrated the compounding effect of an em unit. Because each child element inherits its font-size from its nearest parent that inherits its font-size from its nearest parent (and so on), the final font-size is undesirably 120px.
rem, or root em, was designed to address this issue. To see this in action, replace all the em-child classes with the rem-child class in the index.html file, as seen below:
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <meta http-equiv="X-UA-Compatible" content="IE=edge"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <style> .container { display: flex; justify-content: center; align-items: center; height: 100vh; width: 100wh; } .parent { font-size: 15px; } .em-child { font-size: 2em; } .rem-child { font-size: 2rem; } </style> <title>Document</title> </head> <body> <article class="container"> <div class="parent"> I'm 15px <div class="rem-child"> I'm 15px * 2 = 30px <div class="rem-child"> I'm 15px * 2 = 30px <div class="rem-child"> I'm 15px * 2 = 30px. </div> </div> </div> </div> </article> </body> </html>
This is what we get:
The big question remains: when should you use em vs. rem? There is debate about this, but from what we have learned so far, I recommend using rem for consistency and predictability, and em if you want to scale your page on a modular level.
em and remCreating common decimal property values with em and rem used to be a difficult problem because the math is typically difficult to calculate. The following are some common font sizes and their corresponding rem values:
14px = 0.875rem 16px = 1rem 20px = 1.25rem 24px = 1.5rem 30px = 1.875rem
However, there have been different approaches to resolving this. The “62.5 percent” technique is a popular one; it involves setting the base font-size to 62.5 percent of its original (16px) value, which resets it to 10px (i.e., 62.5 percent of 16 is 10); we can then apply rem/em with more convenient values:
body { font-size: 62.5%; /* 10px */ } h1 { font-size: 2.4rem; /* 24px */ } h2 { font-size: 2rem; /* 20px */ } p { font-size: 1.4rem; /* 14px */ }
The same code applies to em, assuming no other parent elements are overriding these stylings with a different value.
However, there are some significant drawbacks to this approach, particularly in terms of repetition. As in our previous code example, because we’ve now defined the base font size to be 10px, we’ll have to explicitly define font-sizing for every other element that was supposed to inherit from the base. For example, we’ve done this for the p element and will have to do it again for every li, span, and other similar elements that were supposed to inherit this value from base.
em and remIn contrast to em, rem appears to be more common in CSS libraries. Popular libraries that make use of rem include:
The Google Material design style guide also recommends the rem unit for its web implementations.
em and rem are two similar, scalable, and relative CSS units. The key difference between them lies in how browsers compute their pixel values.
While both these units provide the needed flexibility for a responsive design, rem is favored for its simplicity, consistency, and predictability. And although em can be tricky to use, if you like to scale your page on a modular level, it can be the right choice.
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One Reply to "Using <code>em</code> vs. <code>rem</code> in CSS"
None of the design programs support the REM unit. so the next time you feel like using Rem, try the following thought experiment: what if we use 1 pixel as the base unit for rem?. then everyone who wants to scale something there will be able to easily do it , and the rest of the developers will not suffer constantly translating pixels into remes when laying out the layout.