When to Use REM vs PX: A Simple Guide

When to Use REM vs PX: A Simple Guide

If you’ve spent any time writing CSS, you’ve probably stared at your screen and wondered, “Should I make this margin 16px or 1rem?”

It’s one of the most common questions in web design, and honestly, the answer is a lot simpler than most tutorials make it out to be. To figure out when to use which, we first have to talk about how these two units actually behave in the wild.

Meet the Contenders

Think of a pixel (px) like a physical ruler. If you tell a box to be 300 pixels wide, it is going to be exactly 300 pixels wide. It doesn’t care if your user is reading on a massive 4K monitor or squinting at a tiny phone screen. It doesn’t care if the user is legally blind and has cranked their system font size up to 200%. Pixels are stubborn. They do exactly what they are told, no matter what.

On the flip side, think of a rem (which stands for “Root em”) like a multiplier. A rem takes its marching orders from whatever the root font size of the browser is.

By default, almost every browser sets its base font size to 16px. So, 1rem equals 16px. 2rem equals 32px. Simple enough, right? But here is where the magic happens: if a user goes into their browser settings and changes their default text size to 20px because they have bad eyesight, 1rem suddenly becomes 20px. Everything scales gracefully.

The E-Reader Analogy

Imagine buying a printed paperback book. The text is printed at a fixed size. If the words are too small for you to read, tough luck—you need to go buy a magnifying glass. That’s a pixel.

Now imagine reading a book on a Kindle. If the text is too small, you just tap a button to bump up the font size. The words get bigger, the line spacing adjusts, and the margins shift to make room. Everything adapts to your preference. That’s a rem.

The Golden Rule: Accessibility First

The main reason the web development community has heavily shifted toward using rem isn’t just because it’s a cool modern trick. It’s all about web accessibility.

When you hardcode your typography using font-size: 16px;, you are essentially overriding the user’s personal browser settings. You’re handing them the printed paperback and telling them to deal with it.

When you use font-size: 1rem;, you’re handing them the Kindle. You’re respecting their setup.

So, when should I use REM?

As a general rule of thumb, you want to use rem for anything that dictates the general “flow” and readability of your content.

• Typography: Always use rem for font-size.
• Spacing around text: Use rem for margin and padding on text elements (like paragraphs, headings, and buttons). If a user bumps up their font size, you want the breathing room around that text to scale up with it so things don’t look cramped.
• Media Queries: This one surprises people! Setting your breakpoints (like @media (min-width: 48rem)) instead of 768px ensures that if a user has a massive default font size, your mobile layout kicks in sooner so the text doesn’t overflow or break your design.

(Pro tip: If doing the math to figure out what 48px is in rems makes your head spin, don’t sweat it. You can just use our PX to REM Converter available right here on pixelconverter.com to do the heavy lifting for you!)

Are pixels completely dead?

Not at all! Pixels still have a very important job. You just have to know when to deploy them.

You should use px for things that absolutely must not scale, even if the user changes their font size.

• Borders: A 1px solid black border should usually stay a crisp, thin line. If you make it 0.1rem, it might scale up and look like a thick, clunky block when someone increases their font size.
• Shadows: Box shadows (box-shadow) generally look best when they stay fixed to the physical dimensions of the screen, rather than scaling with text.
• Fixed-size images or icons: If you have an avatar or an SVG icon that needs to be exactly 40px by 40px to fit into a specific UI slot without breaking the layout, use pixels.

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The Cheat Sheet

If you’re ever in doubt, just check this quick list:

Use REM for:

• font-size (Always!)
• margin and padding (Especially around text)
• width and height of text-heavy containers
• Media queries (@media)

Use PX for:

• border-width
• box-shadow offsets and blur
• Explicit sizing for small UI elements (like a fixed-size icon)
• Anything that would visually break if it got unexpectedly large

At the end of the day, using rem is about building websites that are flexible, resilient, and respectful of the people actually reading them. Let the pixels handle the tiny decorations, and let the rems handle the heavy lifting!

Frequently Asked Questions (FAQ)

What is the difference between em and rem?

While rem stands for “root em” and scales based on the root <html> font size, a standard em scales based on the font size of its immediate parent container. This means em values can compound and snowball out of control if you nest elements, making rem the much safer and more predictable choice for overall layout and typography.

Should I use the 62.5% CSS trick?

Some developers set html { font-size: 62.5%; } so that 1rem equals exactly 10px (making the math easier for their brains). While it’s a popular hack, it can sometimes interfere with accessibility and third-party plugins. It’s often better to just leave the root font size alone and use a PX to REM converter if the math gets confusing.

Is it okay to mix px and rem?

Absolutely! In fact, you should. The most bulletproof, professional websites use a combination of both: rem for typography, spacing, and layout flexibility, and px for rigid, decorative elements like borders and shadows.