em.scss

// http://viljamis.com/blog/2013/prototyping-responsive-typography/?utm_source=Responsive+Design+Weekly&utm_campaign=76e7785581-Responsive_Design_Weekly_046&utm_medium=email

$browser-context: 16; // Default

@function em($pixels, $context: $browser-context) {
  @return #{$pixels/$context}em
}

example.scss

body {
  font-size: em(16);
}

p.intro {
  font-size: em(24);
}

h1 {
  font-size: em(72);
}