The core difference between rem and em is that the calculation benchmarks are different. Rem is always based on the font size of the root element (usually ), suitable for global consistency scenarios such as basic font size and layout spacing; while em is based on the font size of the recent parent element, suitable for internal use of components that require local scale scaling, such as button icons and form tags; using rem can avoid the size multiplication problem during nesting, and supports overall scaling by adjusting the root font size, while using em requires attention to the unexpected effects that may be caused by hierarchical overlay.
When building responsive and accessible websites, choosing the right unit for sizing elements in CSS matters. Two units that often come up in discussions are em and rem . While they might seem interchangeable at first glance, they behave differently and serve different purposes depending on your layout needs.
What's the basic difference?
The main difference between em and rem lies in what they're based on.
-
emis relative to the font size of its direct or nearest parent element . -
remstands for "root em" and is always relative to the font size of the root element , which is usually theelement.
This means that with em , sizes can compound if you're not careful—especially in deeply nested elements. With rem , everything stays consistent because it's tied directly to the root font size.
For example:
html {
font-size: 16px;
}
div {
font-size: 1.5em; /* 24px */
}
span {
font-size: 1.5em; /* 36px (based on parent div's 24px) */
} In this case, the span ends up much bigger than expected because each level inherits and scales the size.
When to use em
Use em when you want an element's size to scale relative to its parent —this is especially useful for UI components like buttons or cards where internal spacing should stay proportional even if the component's overall size changes.
Some practical uses:
- Icons inside buttons
- Padding/margin within a module or component
- Font sizes for labels inside form elements
Just keep in mind that nesting multiple em values can lead to unexpected results unless you carefully track inherited sizes.
When to use rem
Use rem when you want more predictable and consistent sizing across your site. It's ideal for things like:
- Base font sizes for body text
- Spacing (margins, padding) between major layout elements
- Widths and heights of larger containers
Because rem is always based on the root font size, it avoids compounding issues and makes global design changes easier. For instance, if you change the root font size, all rem -based measures scale accordingly—great for accessibility features like allowing users to adjust default font sizes.
A common practice is setting a base font size like this:
html {
font-size: 16px;
} Then using rem for nearly everything else.
A few small but important details
One thing many developers miss early on is how browser defaults affect rem . Most browsers set the default <html> font size to 16px, so 1rem equals 16px by default—but this can vary if users have changed their settings.
Also, using media queries with rem can help maintain visual consistency. For example:
@media (min-width: 40rem) {
/* Adjust styles for larger screens */
}This way, breakpoints scale along with the root font size, keeping proportions intact.
Another tip: If you're working on a large project, consider using a CSS preprocessor or tool that helps convert pixel values into rem automatically—it reduces manual calculation errors.
Basically that's it. Choosing between em and rem comes down to whether you need local scaling ( em ) or global consistency ( rem ). Both are powerful, just suited for different situations.
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