In the fifteen years since rem was introduced, I've heard many variants of the question:
What CSS units should I be using here,
Now, there are a lot of other options to consider as well:
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.
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
The majority of CSS examples on the internet use
px values because they have been the de facto standard.
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
pt you'd need to use
0.75pt for consistent results, and that's just not very convenient.
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
There are two parts to
rem's advantage over other units:
- User preferences are respected
- You can change the apparent
remto 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
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
Changing Apparent px Value
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
Whatever you do, don't set the
font-size to a
If you set the
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
The math for this is reasonably straight-forward.
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
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
It's important to note, with this adjustment in place, the apparent value of
10px, which means any value you might have written in
px can be converted directly to
rem by bumping a decimal place.
font-size you choose is up to you, so if you want there's no reason you can't use
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.
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
Unfortunately, Safari has a bug where
rem units are recomputed. Fortunately you can use
em units as they also use the initial value of
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.
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.
If you switch between various different projects, it's quite possible that the apparent
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
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
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