Should I use px or rem units in CSS?

In the fifteen years since rem was introduced, I've heard many variants of the question:

What CSS units should I be using here, px or rem?

Now, there are a lot of other options to consider as well:

  • ch
  • cm
  • em
  • ex
  • in
  • lh
  • mm
  • pc
  • pt
  • Q
  • vh
  • vmax
  • vmin
  • vw

Many of these options are useful in a variety of different circumstances, but I'm going to intentionally ignore them for purposes of this post.

Unit Overview

Historically px units typically represented one device pixel. With devices having higher and higher pixel density, this no longer holds for most modern devices.

rem units represent the root em size. It's the font-size of whatever matches :root. In the case of HTML, it's the <html> element; for SVG, it's the <svg> element. The default font-size in every browser is 16px.

Using px

The majority of CSS examples on the internet use px values because they have been the de facto standard. pt, in, and cm could have been used in theory, but they didn't handle small values well as you'd quickly need to resort to fractional units, which were longer to type and harder to reason about.

If you wanted a thin border, with px you could use 1px, with pt you'd need to use 0.75pt for consistent results, and that's just not very convenient.

Using rem

rem's default value of 16px isn't a very strong argument for its use. Writing 0.0625rem is a worse developer experience than writing 0.75pt, so why would anyone use rem?

There are two parts to rem's advantage over other units:

  • User preferences are respected
  • You can change the apparent px value of rem to whatever you'd like

Respecting User Preferences

Browser zoom has changed a lot over the years. Historically, many browsers would only scale up font-size, but that changed pretty rapidly when websites realized that their beautiful pixel-perfect designs were breaking any time someone zoomed in or out. At this point, browsers scale the entire page, so font-based zooming is out of the picture.

Respecting a user's wishes is not out of the picture. Just because a browser's font-size is set to 16px by default, doesn't mean any user can't change their preferences to 24px or 32px to correct for low vision or poor visibility (e.x. screen glare). If you base your units off of rem, any user at a higher font-size will see a proportionally larger site. Borders will be bigger, padding will be bigger, margins will be bigger; everything will scale up fluidly.

If you base your media queries on rem, you can also make sure that the site your users see fits their screen. A user with font-size set to 32px on a 640px wide browser will effectively be seeing your site as shown to a user at 16px on a 320px wide browser. There's absolutely no loss for RWD in using rem.

Changing Apparent px Value

Because rem is based on the font-size of the :root node, if you want to change what 1rem represents, all you have to do is change the font-size:

Whatever you do, don't set the :root element's font-size to a px value!

If you set the font-size on html to a px value, you've blown away the user's preferences without a way to get them back.

If you want to change the apparent value of rem, use % units.

The math for this is reasonably straight-forward.

The apparent font-size of :root is 16px, but let's say we want to change it to 20px. All we need to do is multiply 16 by some value to get 20.

Set up your equation:

16 * x = 20

And solve for x:

x = 20 / 16
x = 1.25
x = 125%

Doing everything in multiples of 20 isn't all that great, but a common suggestion is to make the apparent size of rem equal to 10px. The magic number for that is 10/16 which is 0.625 or 62.5%.

The problem now is that your default font-size for the rest of the page is set way too small, but there's a simple fix for that: Set a font-size on body using rem:

It's important to note, with this adjustment in place, the apparent value of rem is 10px, which means any value you might have written in px can be converted directly to rem by bumping a decimal place.

padding: 20px

turns into

padding: 2rem

The apparent font-size you choose is up to you, so if you want there's no reason you can't use

to have 1rem equal 1px.

So there you have it, a simple solution to respect users' wishes while also avoiding over-complicating your CSS.

Wait, so what's the catch?

I was afraid you might ask that. As much as I'd like to pretend that rem is magic and solves all-the-things, there are still some issues of note. Nothing deal-breaking in my opinion, but I'm going to call them out so you can't say I didn't warn you.

Media Queries

One of the first issues you'll run into with rem involves media queries.

Consider the following code:

Here the value of rem changes depending on whether the media-query applies, and the media query depends on the value of rem, so what on earth is going on?

rem units in media queries use the initial value of font-size and should not take into account any changes that may have happened to the font-size of the :root element. In other words, it's apparent value is always 16px.

Unfortunately, Safari has a bug where rem units are recomputed. Fortunately you can use em units as they also use the initial value of font-size (16px) and work correctly cross-browser.

This can be a bit annoying, because it means that you have to do some fractional calculations, but I have found that most common media queries already use values that are multiples of 16.

px rem
320 20
480 30
768 48
1024 64
1200 75
1600 100

Additionally, if you're using a CSS preprocessor, you can use mixins or variables to manage your media queries, which will mask the issue entirely.

Context Switching

If you switch between various different projects, it's quite possible that the apparent font-size of rem will have different values. In one project you might be using an apparent size of 10px, where in another project the apparent size might be 1px. This can be confusing and cause issues.

My only recommendation here is to stick with font-size: 62.5% to convert rem to an apparent size of 10px, because that has been more common in my experience.

Shared CSS Libraries

If you're writing CSS that's going to be used on a site that you don't control, such as for an embedded widget, there's really no good way to know what apparent size rem will have. If that's the case, feel free to keep using px.

If you still want to use rem though, consider releasing a Sass or LESS version of the stylesheet with a variable to override the scaling for the apparent size of rem.

Recap

In summary, support your users by making sure that their font preferences are respected.

When you need an absolute unit size, use rem. For media queries, use em.