Edit (2014/11/16): I have changed my mind again and no longer use Sass to “remify”. In most projects, I’ve noticed it’s better to use a postprocessor such as px_to_rem.

Edit (2014/05/13): this article is getting old and while it still is perfectly valid, I have kind of changed my mind about this whole px to rem thing. I now use something simpler, like this mixin.

About REM

Everybody loves relative units. They are handy and help us solve daily problems. However the most used one ( em ) presents some issues, especially when it comes to nesting.

As an example, setting both p and li tags font-size to 1.2em may seem fine. But if you ever happen to have a paragraph inside a list item, it would result in a font-size 1.44 times (1.2 * 1.2) bigger than parent font-size, and not 1.2 as wished.

To avoid this, a new unit has been created: rem . It stands for root em. Basically, instead of being relative to the font-size of its direct parent, it’s relative to the font-size defined for the html element.

You may have already seen something like this in frameworks, demo, blog posts and such:

html { font-size : 62.5% ; } body { font-size : 1.6rem ; }

Because all browsers have a default font-size of 16px , setting the font-size to 62.5% on the html element gives it a font-size of 10px (10 / 16 * 100 = 62.5) without explicitely setting it to 10px which would prevent zooming. Then, setting a font-size of 1.6rem on the body element simply results in a font-size of 16px , cascading through the whole DOM tree.

Then, if I want an element to have like a 28px font-size, I simply have to do .element { font-size: 2.8rem; } , no matter the size of its parent.

Everything is great, however rem isn’t supported in all browsers, especially not in Internet Explorer 8, which is still required in most projects. It means we have to give a fallback for this browser.

Mixin to the rescue!

Having to define twice the font-size property everytime you have to set the size of a text element sucks. This is the moment you’d like to have a wonderful mixin handling everything for you. Well, WISH GRANTED!

About the mixin

There are already many mixins handling px fallback for rem usage, most of them do it very well. However this one pushes things a step further. It is inspired by this rem mixin by Hans Christian Reinl, revamped by myself to make it even more awesome. Here are the features:

Accepts either px or rem as an input value

or as an input value Accepts (almost) any property as an input, not only font-size

Accepts multiple values, like 10px 20px (for padding or margin as an example)

Let’s open the beast

html { font-size : 62 .5% ; /* 1 */ } @function parseInt ( $n ) { /* 2 */ @return $n / ( $n * 0 + 1 ); } @mixin rem ( $property , $values ) { $px : (); /* 3 */ $rem : (); /* 3 */ @each $value in $values { /* 4 */ @if $value == 0 or $value == auto { /* 5 */ $px : append ( $px , $value ); $rem : append ( $rem , $value ); } @else { $unit : unit ( $value ); /* 6 */ $val : parseInt ( $value ); /* 6 */ @if $unit == 'px' { /* 7 */ $px : append ( $px , $value ); $rem : append ( $rem , ( $val / 10 + rem )); } @if $unit == 'rem' { /* 7 */ $px : append ( $px , ( $val * 10 + px )); $rem : append ( $rem , $value ); } } } @if $px == $rem { /* 8 */ #{ $property } : $ px ; /* 9 */ } @else { #{ $property } : $ px ; /* 9 */ #{ $property } : $ rem ; /* 9 */ } }

This may be a bit rough so let me explain it:

The mixin relies on a baseline of 10px The mixin relies on a function to parse the integer from a value with a unit We define a list of values for both units We iterate through each value in the given parameter $values If the value is either auto or 0 , we append it to the list as-is If the value has a unit, we split it to get both the unit and the raw value We append according values to the lists depending on the unit of the given value If the two lists are the same, we ouput only one (like margin-top: 0 ) We output the result

Thanks to Moving Primates to improve the mixin by adding step 8. ;)

Usage

Using it is pretty straightforward:

html { font-size : 62 .5% ; } body { @include rem ( font-size , 1 .6rem ); @include rem ( padding , 20px 10px ); }

… outputs:

html { font-size : 62.5% ; } body { font-size : 16px ; /* Fallback for IE8 */ font-size : 1.6rem ; padding : 20px 10px ; /* Fallback for IE8 */ padding : 2rem 1rem ; }

Remaining issues

There are still some issues with this mixin:

Doesn’t work with all properties (border shorthand among others)

Doesn’t fallback if you input a wrong value (wrong unit or unitless value as an example)

Relies on a defined baseline; however this is easily fixed by adding a $baseline parameter to the mixin

parameter to the mixin Relies on a parseInt() function; I’ve proposed it to Compass, let’s hope they add it anytime soon

If you ever happen to find a decent solution to fix one, I’ll be glad to know and add it!

Final words

That’s pretty much it folks. I’d be glad to hear your opinion on this and improve it with your ideas. :)

If you want a playground to test and hack, please feel free to fork my pen.