In modern web design, using relative units like rem is often seen as a best practice. Rem units offer a flexible, scalable way to ensure that your web layouts adapt smoothly across different screen sizes and devices. They allow elements to scale relative to the root font size, making it easier to maintain consistency and improve accessibility for users who may need to adjust font sizes for readability. However, while rem units can be powerful, they can also introduce unexpected challenges if used incorrectly.
This article explores the pitfalls of using rem units and how to avoid common mistakes when implementing them. We’ll cover key concepts about rem units, examine real-world examples where they can backfire, and provide strategies for ensuring your designs remain both accessible and scalable without unintended side effects.
Understanding Rem Units
To begin, let’s break down what rem units are and why they’re commonly used in web development.
Rem (root em): A rem unit is a relative unit of measurement in CSS. Unlike the em unit, which is relative to the font size of its closest parent element, rem units are always relative to the root element’s font size (usually the <html>
element). By default, the root font size is 16px in most browsers, but it can be changed using CSS.
For example, if the root font size is set to 16px:
1rem
= 16px2rem
= 32px
Because rem units scale based on the root font size, they offer more predictable behavior than em units, which can compound with each parent’s font size and lead to unexpected results. This is why rem units are often recommended for responsive design, where consistency and scalability are key.
The Benefits of Rem Units
Before diving into the potential issues with rem units, let’s briefly highlight why they’re so widely used.
Accessibility: Rem units scale with the browser’s font settings, making it easier for users to adjust text sizes for readability without breaking the layout. If a user increases the default font size in their browser for accessibility, rem-based elements will scale proportionally.
Consistency: Unlike em units, which change based on the parent element’s size, rem units are always based on the root font size, providing a more consistent layout across different components.
Responsive Design: Rem units allow for scalable designs that adapt to different devices and screen sizes without hardcoding specific pixel values.
While rem units provide these advantages, they are not a one-size-fits-all solution. In certain situations, using rems can cause unexpected design issues. Let’s explore some common scenarios where rem units can backfire and how to address these challenges.
Pitfall #1: Unexpected Scaling Due to Root Font Size Changes
One of the most common issues with rem units is that they scale based on the root font size. This can lead to problems if you change the root font size dynamically or if the root size is altered by user preferences.
The Problem:
Let’s say you design your layout based on the assumption that the root font size is 16px. If the root font size is increased to 20px (either programmatically or because the user has set their browser to use a larger font size), all elements sized with rem units will scale accordingly. This can result in a layout that looks too large or breaks the intended design.
For example:
html {
font-size: 16px;
}
h1 {
font-size: 2rem; /* 32px */
}
p {
font-size: 1rem; /* 16px */
}
If the root font size changes to 20px, then:
h1
will become 40px (2rem * 20px)p
will become 20px (1rem * 20px)
While this scaling is beneficial for accessibility, it can create problems if the rest of your design (margins, paddings, and layout) doesn’t scale proportionally, leading to overly large text and broken layouts.
The Fix:
To avoid unexpected scaling issues, it’s crucial to test your design with different root font sizes. Additionally, you can use rem units selectively. For example, use rem for text sizing where accessibility scaling is important, but use relative percentage-based units (like %
or vw/vh
) for layouts that need to remain consistent regardless of text size changes.
/* Adjust font size for body text but use % for layout */
body {
font-size: 1rem; /* Will scale with root size */
}
.container {
width: 80%; /* Layout won't change based on root size */
}
By combining rem units for typography and percentage-based units for layouts, you maintain flexibility without risking an inconsistent design.
Pitfall #2: Misalignment Between Text and Layout Elements
Another challenge with rem units is when they affect more than just text. If rem units are used for margins, paddings, and spacing, changing the root font size can lead to a misalignment between text and other layout elements.

The Problem:
Imagine you’re using rem units to define both your text size and your layout’s spacing. If the root font size changes, both the text and the layout will scale. However, this can create issues if the scaling isn’t proportional or if certain elements (like images or buttons) are fixed in size and don’t scale with the root font size.
html {
font-size: 16px;
}
.button {
font-size: 1rem; /* 16px */
padding: 1rem 2rem; /* 16px top/bottom, 32px left/right */
border-radius: 0.5rem; /* 8px */
}
If the root font size changes, the padding and border radius will scale along with the text, which can lead to buttons or other elements looking too large or small compared to other parts of the layout.
The Fix:
Use rem units for typography but use fixed units (like pixels) or percentage-based units for spacing and layout elements that don’t need to scale with text size. This ensures that key interactive elements, like buttons, remain proportional even when the text size changes.
.button {
font-size: 1rem; /* Scales with root size */
padding: 10px 20px; /* Fixed padding */
border-radius: 8px; /* Fixed border radius */
}
By doing this, you allow the text to scale for accessibility without causing buttons, padding, or layout elements to balloon out of proportion.
Pitfall #3: Complications in Complex Layouts with Nested Elements
In complex layouts with nested elements, using rem units can introduce subtle layout issues, especially when combined with other responsive techniques like media queries or flexbox. The uniform scaling of rems can sometimes cause elements to shift unexpectedly, leading to misalignment or spacing inconsistencies.
The Problem:
Consider a layout where you’re using rem units for both font sizes and spacing. In a nested flexbox layout, if the root font size changes, the child elements might scale in a way that causes unintended layout shifts.
html {
font-size: 16px;
}
.container {
display: flex;
gap: 2rem; /* 32px gap between items */
}
.item {
font-size: 1.5rem; /* 24px */
padding: 1rem; /* 16px padding */
}
If the root font size increases, the gap and padding also increase, which might cause the items to spread out more than intended, especially on smaller screens.
The Fix:
When working with complex layouts, consider using rem units only for typography and relying on flexbox or grid’s built-in spacing controls for layout consistency. Additionally, use rems in combination with media queries to fine-tune your design for different screen sizes.
.container {
display: flex;
gap: 20px; /* Fixed gap */
}
@media (min-width: 600px) {
.container {
gap: 40px; /* Increase gap for larger screens */
}
}
.item {
font-size: 1.5rem; /* Scales with root size */
}
By controlling the layout independently of the font size, you can avoid unintended spacing issues in responsive designs.
Pitfall #4: Over-Reliance on Rem Units in Media Queries
Using rem units in media queries can lead to confusing behavior, especially when the root font size changes. Since rem units are based on the root font size, changing the root size will alter the breakpoints of your media queries, which can lead to inconsistent behavior across devices.
The Problem:
If your media queries use rem units and the root font size changes (either through user preferences or programmatically), the breakpoints will shift, potentially causing layouts to break or render inconsistently.
html {
font-size: 16px;
}
@media (min-width: 40rem) {
/* Breakpoint at 640px if root size is 16px */
.container {
display: flex;
}
}
If the root font size changes to 20px, then 40rem
becomes 800px, meaning the breakpoint now applies at a larger screen width, potentially causing the layout to break or shift unexpectedly.
The Fix:
When defining media queries, it’s safer to use pixel-based units for breakpoints to ensure consistent behavior, regardless of changes in the root font size.
@media (min-width: 640px) {
.container {
display: flex;
}
}
By using fixed units like pixels for breakpoints, you can maintain predictable responsive behavior across different devices and browser settings, ensuring your layout behaves consistently even when users adjust their font size settings.
Pitfall #5: Accessibility Trade-Offs
While rem units are often promoted for their accessibility benefits, they can introduce trade-offs if not used correctly. Scaling everything based on the root font size can make it difficult to fine-tune the accessibility of individual elements, especially when users need larger text but don’t want the entire layout to scale.
The Problem:
For users with visual impairments, increasing the text size using rem units may inadvertently scale other elements like buttons, padding, and layout margins, potentially making the design harder to navigate. Additionally, overly large elements may cause frustration if users only want larger text without scaling other parts of the UI.
The Fix:
To enhance accessibility without compromising the layout, use rem units for text sizes but rely on other units (like pixels or percentages) for layout and spacing. This ensures that text scales appropriately for users with visual impairments, while the overall layout remains consistent.
body {
font-size: 1rem; /* Scales with user preferences */
}
.container {
width: 90%; /* Consistent layout */
padding: 20px; /* Fixed padding */
}
By decoupling typography from layout, you ensure that users who adjust font sizes for accessibility will see larger text without breaking the visual design of your website.
Advanced Strategies for Managing Rem Units in Complex Designs
Once you’ve got a solid understanding of the basic pitfalls of rem units, the next step is to explore advanced strategies that will help you avoid problems in more complex, component-based designs. Web development today often involves frameworks, dynamic layouts, and modular design systems where rem units are frequently used to create scalable, reusable components. However, the more intricate the design, the more careful you need to be about how rem units are applied. Let’s dive deeper into how to manage rem units effectively in sophisticated, real-world web projects.

1. Using Rem Units in Modular Component Systems
One of the key advantages of using rem units is their ability to create scalable, modular components. When building a design system or a component library, it’s common to use rem units to ensure that typography scales consistently across different components. However, applying rem units to every property—such as padding, margins, or even element widths—can lead to inconsistent results when those components are used in different contexts.
The Problem:
When rem units are applied to every part of a component (including padding, margins, and width), the entire component’s size scales based on the root font size. This is generally a good thing for text-based elements, but when components are nested or combined in complex layouts, the unintended scaling of non-typographic properties can lead to spacing inconsistencies and layout-breaking side effects.
For example, consider a card component that uses rem units for padding, margins, and width:
.card {
padding: 2rem;
margin-bottom: 2rem;
width: 20rem;
}
This approach ensures the card scales when the root font size changes, but if the card is nested inside a larger container that also uses rem-based padding and margins, the scaling can quickly get out of hand.
The Fix:
To avoid excessive scaling, use rem units selectively within modular components. Apply rem units for typography and text-related padding while using percentage-based or fixed units (such as pixels) for layout-related properties like width and margins.
Here’s an optimized approach:
.card {
padding: 1rem 2rem; /* Use rem for text-related padding */
margin-bottom: 20px; /* Use fixed units for layout spacing */
width: 90%; /* Use percentage for layout scaling */
}
By combining rem units with percentage-based and fixed units, you can ensure that components remain scalable while avoiding unexpected layout shifts. This balance between flexibility and control is key to building modular, reusable design systems.
2. Combining Rem Units with CSS Grid and Flexbox
CSS Grid and Flexbox have become essential tools for creating responsive layouts. While these layout systems are powerful on their own, combining them with rem units can introduce more complexity if not handled carefully.
The Problem:
Using rem units for layout properties (like gap, width, or grid template columns) in grid or flexbox layouts can lead to problems when the root font size changes. Because both Grid and Flexbox are responsive by design, their layouts are often based on the available space rather than a fixed size. When rem units are applied directly to the layout properties, the scaling can become unpredictable, especially on larger or smaller screens.
For example, applying rem units for the gap in a grid layout:
.grid-container {
display: grid;
grid-template-columns: repeat(3, 1fr);
gap: 2rem; /* Gap scales with root font size */
}
If the root font size changes, the gap between grid items will also change, which might not always be desirable.
The Fix:
Use relative units like percentages or viewport units (like vw
or vh
) for layout properties in Grid and Flexbox, and reserve rem units for typography and text-related padding. This ensures that the grid or flexbox layout scales according to the available space, while text and inner padding scale based on the root font size.
.grid-container {
display: grid;
grid-template-columns: repeat(3, 1fr);
gap: 20px; /* Use fixed or percentage-based units for layout */
}
.item {
font-size: 1rem; /* Use rem for typography */
padding: 1rem; /* Use rem for text-related padding */
}
This approach keeps the layout consistent while allowing text to scale for accessibility.
3. Using Media Queries with Rem Units
When applying rem units inside media queries, it’s important to remember that breakpoints defined using rem units will shift if the root font size changes. This can be useful if you want responsive behavior to scale with the user’s font settings, but it can also lead to unexpected results if you’re not careful.
The Problem:
If you define breakpoints using rem units and the root font size changes (either because of user preferences or design adjustments), the breakpoints will also shift, potentially leading to unintended layout changes. For example, a media query defined like this:
@media (min-width: 50rem) {
.container {
display: flex;
}
}
If the root font size is 16px, this breakpoint triggers at 800px (50rem * 16px). But if the root font size increases to 20px, the breakpoint moves to 1000px, which could disrupt your responsive design.
The Fix:
To maintain consistent breakpoints across devices and font sizes, use fixed units like pixels for your media queries, especially for defining key breakpoints. You can continue using rem units inside the media query for font size and padding adjustments, but the breakpoint itself should be static to avoid unexpected shifts.
@media (min-width: 800px) {
.container {
display: flex;
}
.item {
font-size: 1.5rem; /* Use rem for typography */
}
}
This ensures that your layout remains responsive based on screen size, not the root font size, while still allowing for typography scaling within the breakpoints.
4. Working with Rem Units in Accessibility-First Design
Designing with accessibility in mind is crucial, and rem units are often recommended as part of an accessibility-first approach. However, there’s a fine line between making text scalable and over-scaling other elements, especially for users who increase their font size settings.
The Problem:
When rem units are applied indiscriminately across an entire design, increasing the root font size can cause everything (including buttons, margins, and layout components) to scale. While this benefits accessibility for text, it can lead to overly large buttons, excessive padding, or awkward spacing, reducing the overall usability of the interface.
For example, if a user sets their browser’s root font size to 20px, and your design relies heavily on rem units for both text and layout, buttons and other interactive elements may become too large and hard to navigate:
.button {
font-size: 1.2rem; /* Scales with root size */
padding: 1rem; /* Also scales with root size */
}
The Fix:
For accessibility-first design, focus on using rem units for font sizes and related padding, but use fixed or percentage-based units for layout, margins, and button sizes. This allows users to adjust text sizes for readability without affecting the overall layout or interaction elements.
.button {
font-size: 1.2rem; /* Scales with user preferences */
padding: 10px 20px; /* Fixed padding */
width: 200px; /* Fixed or percentage-based width */
}
This ensures that buttons and other interactive elements remain accessible and easy to use, even when text sizes are increased for readability.
5. Testing Rem-Based Designs Across Devices and User Settings
Rem units are influenced by both the default root font size and any changes users make to their browser settings. It’s essential to test your design across a variety of devices, browsers, and user settings to ensure that your layout remains consistent and accessible.
The Problem:
While rem units are great for ensuring flexibility, relying on them too heavily can result in inconsistent behavior across devices, especially if users have set different default font sizes or have enabled accessibility features that modify font scaling. What looks perfect on your development machine may break on a user’s device with a larger or smaller root font size.
The Fix:
Regularly test your designs with different root font sizes, screen resolutions, and browser settings. Use browser dev tools to simulate different font sizes and accessibility preferences, ensuring that your layout doesn’t break when font sizes are increased or decreased.
In Chrome DevTools, for example, you can adjust the font size settings to see how your rem-based design scales across different environments. This helps you identify potential issues with scaling and adjust your design accordingly.
Conclusion: Using Rem Units Wisely
Rem units are an invaluable tool in modern web design, offering flexibility, scalability, and accessibility benefits that make them a popular choice for building responsive websites. However, as with any tool, rem units can lead to unexpected issues if not used correctly. By understanding the pitfalls of rem units—such as unexpected scaling, layout misalignments, and accessibility challenges—you can create more robust, adaptable designs that function smoothly across different devices and user preferences.
To avoid these pitfalls:
- Use rem units primarily for text sizing and accessibility improvements.
- Combine rems with other units, such as pixels or percentages, for layouts and spacing.
- Test your design across a variety of root font sizes and screen sizes to ensure consistency.
- Use fixed units for media queries to avoid shifting breakpoints.
At PixelFree Studio, we believe in creating designs that are not only visually stunning but also functional and accessible. By mastering the use of relative units like rem, you can build websites that are scalable, flexible, and inclusive—ensuring that every user enjoys a seamless experience, regardless of their device or settings.
Read Next: