font-size in rem and a tool for absolute to em conversion. http://css-tricks.com/snippets/css/less-mixin-for-rem-font-sizing/

r-em-sizing.scss

// Inspired by this post from CSS-Tricks.
// http://css-tricks.com/snippets/css/less-mixin-for-rem-font-sizing/
//
// Plays nice with compass/typography/vertical_rhythm
// http://compass-style.org/reference/compass/typography/vertical_rhythm/
//
// Calculates font size in `rem` (root em).
//
// Relative values depends on $base-font-size. Pixle value for font-size depends
// on $legacy-support-for-ie6 or $legacy-support-for-ie7 or $legacy-support-for-ie8
// being true.
//
// Includes function `rel` to calculate from `px` to `em`. Second argument used
// as a base for conversion. Defaults to $base-font-size.
//
// Why not just calculate everything in rem? Because it's lazy and it will litter
// your styles with pixel values when IE compatibility is needed.
//
// Note: You can use any absolute unit (cm, mm, in, pt and pc) instead of px but
// they are not very practicle.
//
// SCSS:
//
// $base-font-size:             12px;
// $base-line-height:           18px;
//
// .foo {
//   @include font-size(24px);
//   padding: rel(1px 2px 3px 4px, 24px);
// }
//
// CSS:
// 
// .foo {
//   font-size: 24px;
//   font-size: 2rem;
//   line-height: 1.5em;
//   padding: 0.04167em 0.08333em 0.125em 0.16667em;
// }
//

@import "compass/typography/vertical_rhythm";

// Sets the font-size and line-height in rem (root em).
@mixin font-size($font-size) {
  @if unit($font-size) == '' or relative-unit($font-size) {
    @warn "$font-size = #{$font-size} - Requires a numeric value with an absolute unit of measurement.";
  }

  $line-height: $font-size * strip-unit(rhythm(lines-for-font-size($font-size), $font-size));

  @if $legacy-support-for-ie6 or $legacy-support-for-ie7 or $legacy-support-for-ie8 {
    font-size: $font-size;
  }
  font-size: to-relative($font-size, $base-font-size, 'rem');
  line-height: to-relative($line-height, $font-size, 'em');
}

// Convert absolute values to a relative em values.
@function rel($numbers, $relative-to: $base-font-size) {
  $results: ();
  @each $number in $numbers {
    $results: append($results, to-relative($number, $relative-to, 'em'));
  }
  @return $results;
}

// Converts an absolute value to a relative one.
@function to-relative($number, $relative-to, $output-unit) {
  @each $arg in (('$number' $number) ('$relative-to' $relative-to)) {
    @if type_of(nth($arg, 2)) != number or relative-unit(nth($arg, 2)) {
      @warn "#{nth($arg, 1)} = #{nth($arg, 2)} - Only absolute numeric values are accepted.";
    }
  }
  @if $output-unit != "em" and $output-unit != "rem" and $output-unit != "%" {
    @warn "$output-unit = #{$output-unit} - Only 'em', 'rem' or '%' are accepted.";
  }

  $result: strip-unit($number / $relative-to);
  @if $output-unit == "%" {
    $result: $result * 100;
  }
  @return unquote($result + if($result != 0, $output-unit, ''));
}

// Returns numeric values without a unit of measure.
// @see https://github.com/nex3/sass/issues/533#issuecomment-11675408
@function strip-unit($number) {
  @return $number / ($number * 0 + 1);
}