Sass function and mixin to use rem units with optional pixel fallback.

Breaking change in 3.0: now using Sass Modules, using @use and rem is renamed to rem.convert. You could still use @import with no changes (see usage below), but if you need LibSass and Ruby Sass support (both deprecated), you should stay on 2.0 (which works fine) or use the PostCSS version.

Breaking change in 2.0: $rem-fallback is now set to false (see support) and $rem-baseline to 16px by default.

Compatibility: Dart Sass only.

PostCSS version: https://github.com/pierreburel/postcss-rem

See also: https://github.com/pierreburel/sass-em

Install with Yarn or npm:

• yarn add sass-rem
• npm install sass-rem

Import in your project depending of your setup:

@use "rem";
// or @use "~sass-rem" as rem;
// or @use '../node_modules/sass-rem' as rem;

.demo {
  font-size: rem.convert(24px); // Simple
  padding: rem.convert(5px 10px); // Multiple values
  border-bottom: rem.convert(1px solid black); // Multiple mixed values
  box-shadow: rem.convert(0 0 2px #ccc, inset 0 0 5px #eee); // Comma-separated values
  // Multiple properties
  @include rem.convert((
    margin: 10px 5px,
    text-shadow: (1px 1px #eee, -1px -1px #eee) // Parentheses needed because of comma
  ));
}

Will output:

.demo {
  font-size: 1.5rem;
  padding: 0.3125rem 0.625rem;
  border-bottom: 0.0625rem solid black;
  box-shadow: 0 0 0.125rem #ccc, inset 0 0 0.3125rem #eee;
  margin: 0.625rem 0.3125rem;
  text-shadow: 0.0625rem 0.0625rem #eee, -0.0625rem -0.0625rem #eee;
}

It was.

But You can change the namespace to something shorter and use rem function and mixin instead of convert:

@use "rem" as to; // Because why not?

.demo {
  font-size: to.rem(24px);
}

Or you can even load the library globally (but beware of conflicts, avoided by the idea of modules):

@use "rem" as *;

.demo {
  font-size: rem(24px);
}

And if you just don't want to use Sass Modules, you can still use @import with rem function, mixin and namespaced $rem-* variables as before:

@import "sass-rem";

$rem-baseline: 10px;

.demo {
  font-size: rem(24px);
}

You can enable pixel fallback by setting $fallback to true, but you will have to use the mixin instead of the function. The mixin accepts a map to convert multiple properties at once too:

@use "rem" with (
  $fallback: true
);

.demo {
  @include rem.convert(font-size, 24px); // Simple
  @include rem.convert(padding, 5px 10px); // Multiple values
  @include rem.convert(border-bottom, 1px solid black); // Multiple mixed values
  @include rem.convert(box-shadow, 0 0 2px #ccc, inset 0 0 5px #eee); // Comma-separated values
  // Multiple properties
  @include rem.convert((
    margin: 10px 5px,
    text-shadow: (1px 1px #eee, -1px -1px #eee) // Parentheses needed because of comma
  ));
}

Will output:

.demo {
  font-size: 24px;
  font-size: 1.5rem;
  padding: 5px 10px;
  padding: 0.3125rem 0.625rem;
  border-bottom: 1px solid black;
  border-bottom: 0.0625rem solid black;
  box-shadow: 0 0 2px #ccc, inset 0 0 5px #eee;
  box-shadow: 0 0 0.125rem #ccc, inset 0 0 0.3125rem #eee;
  margin: 10px 5px;
  margin: 0.625rem 0.3125rem;
  text-shadow: 1px 1px #eee, -1px -1px #eee;
  text-shadow: 0.0625rem 0.0625rem #eee, -0.0625rem -0.0625rem #eee;
}

You can totally disable rem units by setting $px-only to true (for a lt-ie9 only stylesheet for example):

.demo {
  font-size: 24px;
  padding: 5px 10px;
  border-bottom: 1px solid black;
  box-shadow: 0 0 2px #ccc, inset 0 0 5px #eee;
  margin: 10px;
  text-shadow: 1px 1px #eee, -1px -1px #eee;
}

By default, sass-rem now uses a 16px baseline, but you can change this value with $baseline and by using the baseline mixin on the html element to adjust the root font size. The rem function and mixin will calculate rem values accordingly. For example, you can set $baseline to 10px to have a root font size of 62.5% and improve readability (10px = 1rem), which was the pre-2.0 behavior:

@use "rem" with (
  $baseline: 10px
);

html {
  @include rem.baseline;
}

.demo {
  font-size: rem.convert(24px);
}

Will output:

html {
  font-size: 62.5%;
}

.demo {
  font-size: 2.4rem;
}

You can also change the baseline zoom by passing the desired zoom to the baseline mixin which will calculate it depending of $baseline. Useful for creating responsive typography depending on viewport, especially with a different baseline than 16px:

@use "rem" with (
  $baseline: 10px
);

html {
  @include rem.baseline; // Default zoom to 100%

  @media (max-width: 400px) {
    @include rem.baseline(75%);
  }

  @media (min-width: 800px) {
    @include rem.baseline(125%);
  }
}

Will output:

html {
  font-size: 62.5%;
}

@media (max-width: 400px) {
  html {
    font-size: 46.875%;
  }
}

@media (min-width: 800px) {
  html {
    font-size: 78.125%;
  }
}