Using rem and em units in email

There are a number of different units that can be used to set sizes in CSS these can be grouped into absolute and relative.

Absolute units don’t ever change and are good for consistency but lack flexibility. These include px(pixels), pt(points), pc(picas), cm(centimetres), in(inches) and more.

Relative units are more flexible as they are based of something else. These include vw(viewport width), vh(viewport height), lh(current line height) and more but the ones we’re interested in for accessibility are rem and em.

There are also keyword values which are a bit of a mix of the relative and absolute. medium is the default (equal to 1rem), then small large x-large xx-large etc. are all relative to that. Then smaller and larger will adjust relative to the parent font size.

What are EM units and REM units

em units are equal to the current font size, the name comes from the width of an uppercase M. So if you have a font-size:16px set then 1em would be equal to 16px. If you were then to change that to font-size: 20px for a heading, 1em would be equal to 20px.

rem units are similar to em however these values are relative to the :root font size rather than the current font size so the value doesn’t change.. The name comes from Root EM.

Why you should be using EM and REM units

This root font size can be changed by the user, this is done in the email clients, browser or operating system preferences. So if someone finds small text hard to read they can make it larger in the user settings and it will apply across the whole email.

This only works if we respect their settings and base our font-size on the rem set by the user.

What is the default value of 1rem / medium

In the majority of web browsers the default value is 16px as this is a good readable font size for most people.

However, the default size in clients tends to vary a bit more. Looking at popular email clients it ranges between 12px and 17px

Default font size for email clients

This is just to be used as a rough guide to show the range of sizes found. I have seen a few inconsistencies between tests I’ve run locally and via Litmus.

I’m looking at 3 factors here;

• Default this is the default value in the user setting, equivalent to using font-size: medium
• Root this is the root font-size, mostly this woudlbe the same as the default but could be overridden by the email client style’s, equivalent to using font-size: 1rem
• Inherited this is inherited from the email client styles, equivalent to adding plain text without any stylijng

It’s worth noting that some of these defaults do change with the user settings already.

^* Samsung sets a minimum font-size of 17px so these values of 16px only work when used to generate font-size greater than or wqual to 17px. When used for other setting such as padding, margin, etc. there is no minimum size.

Resetting the font-size

Unfortunately rem units don’t yet have full support in email clients so for now I’m recommending using em units inside the email. But also setting a default font-size on the parent wrapping element to improve consistency and accessibility.

Ideally we want to return the font size to 1rem however some email clients don’t support rem and in some the default value can be very small (12px in Applemail) so we will set a minimum and some fallbacks.

<div style="font-size:medium; font-size:max(16px, 1rem)">

I’m going to look at these values in reverse order as that’s what the priority order is;

font-size:max(16px, 1rem)

Where it’s supported this will reset the font size to user preference 1rem with a minimum size of 16px to adjust for small default root sizes.

The wording is confusing as we’re using max to set a minimum font-size but the idea is it’s picking the larger of the values set here.

N.B This will prevent the user from being able to set a smaller preferred font size. So it’s not an ideal solution, you may want to leave this part off.

font-size:medium

If the email client doesn’t support max we will fall back to using font-size:medium, all email clients I’ve tested support this, and it passed the users prefered font-size.

Edit - February 2022: On a previous version of this article I use rem units and an extra fallback to 16px font-size:16px;font-size:1rem; font-size:max(16px, 1rem) but later found font-size:medium has better support.

Edit - May 2022: It looks like the CSS spec has updated so that font-size: 'medium' is no lnoger required to match the user’s preferred font size so this behaviour may change in the future

Converting your code to use em

Mostly the standard size for 1rem is 16px so using that as a base will keep your sizing looking the same for most users. So to calculate em from px, divide your px value by 16 and that will give you your em value.

So for example

• if your font is 20px divided by 16 gives you 1.25em.
• If you font is 18px divided by 16 give you 1.125em
• If your font is 10px it’s too small. Don’t use 10px fonts.

Ideally your minimum font size would be 1em but at a push I’ll sometimes go down to .8em if you asks really nicely and I don’t have time to talk you round.

What styles should use em

So far we’ve been talking about font-size but you can use em anywhere you can use px. The most important ones to consider are things that affect spacing around the text.

line-height is probably the most important as that should be relative to the font-size for that you can use em (also % works exactly the same) or unitless values are similar except they also inherit as relative, where em and % inherit as fixed value..

margin and padding around text should also be set as em as that can affect the readability. Elsewhere it’s not so important.

Setting width on text using em can help accessibility too. For example <p style="max-width:30em"> would stop small fonts creating very long paragraphs, which are harder to read.

For media queries, if your breakpoints are defined by how the content breaks, then consider how font-size may affect that and if using em would work better than px, although be aware of the potential media queries issue which can clash with out minimum font-size reset.

There’s certainly a good argument for moving everything to em if a user has selected a larger font then they may well want larger images, and larger layout too and it makes the design more consistent. However there may be a reason they have chosen to change the font rather than adjusting the screen resolution or using a zoom tool.

Inherited values

As I mentioned the value of 1em can change through the email. This can be seen as both a pro and a con of using em units.

<h1 style="font-size:2em; margin:1em 0">Heading</h1>
<p style="font-size:1em; margin:1em 0">Paragraph</p>

In this example if the user root font is set to 16px then the paragraph has a margin of 16px 0 but the heading is 32px 0 even though it’s the same code. It’s likely you’d want more spacing around a heading but it can take a while to adjust to it.

Another good example if we have an email with a highlighted section where we want to increase the font-size. We can use the same module we have for other sections but just wrap it with font-size:1.5em and it will increase all the sizes of that section with just a tiny bit of code. The same could be done on a footer where we might want a small font, we can just use font-size: 0.8em and it will reduce the sizes of that section.

Issues

Media queries

If you are setting media queries using rem or em they are always bassed off the users prefered font-size. This will clash with the minimum font size fix we use max(16px, 1rem).

So for example we set a media query of 100em. In Apple Mail with default settings that will be 1200px, but in most other places it would be 1600px.

Attributes

em units don’t work inside attributes, the unit is effectively removed from the code and rendered as if no unit were used, which in defaults to px.

So setting width="10em" with render as 10px not 10em. But this is a minor issue as we can set styles in the style attribute which does support em.

VML

VML has the same issue as attributes, even if you set a size in a style attribute it will still render as px

So setting style="width:10em" with render as 10px not 10em.