rem-mixin.scss

/*
 * a small mixin for easy use of rem with px as fallback
 * usage: @include x-rem(font-size, 14px)
 * usage: @include x-rem(marign, 0 12px 2 1.2)
 * usage: @include x-rem(padding, 1.5 24px)
 *
 * thanks to Eric Meyer for https://github.com/ericam/susy
 * and Hans Christian Reinl for http://drublic.de/blog/rem-fallback-sass-less/
 */
@mixin x-rem($property, $values) {
  // Create a couple of empty lists as output buffers.
  $base-font-size: 16px; // should be consistent with your html/body font-size
  $px-values: ();
  $rem-values: ();

  // Loop through the $values list
  @each $value in $values {
    // For each property value, if it's in rem or px, derive both rem and
    // px values for it and add those to the end of the appropriate buffer.
    // Ensure all pixel values are rounded to the nearest pixel.
    @if $value == 0 or $value == 0px {
      // 0 -- use it without a unit
      $px-values: join($px-values, 0);
      $rem-values: join($rem-values, 0);
    } @else if type-of($value) == number and not unitless($value) and (unit($value) == px) {
      // px value given - calculate rem value from base-font-size
      $new-rem-value: $value / $base-font-size;
      $px-values: join($px-values, round($value));
      $rem-values: join($rem-values, #{$new-rem-value}rem);
    } @else if type-of($value) == number and not unitless($value) and (unit($value) == "%") {
      // % value given - don't add px or rem
      $px-values: join($px-values, #{$value});
      $rem-values: join($rem-values, #{$value});
    } @else if $value == auto {
      // auto - don't add px or rem
      $px-values: join($px-values, auto);
      $rem-values: join($rem-values, auto);      
    } @else {
      // unitless value - use those directly as rem and calculate the px-fallback
      $px-values: join($px-values, round($value * $base-font-size));
      $rem-values: join($rem-values, #{$value}rem);
    }
  }

  // output the converted rules
  #{$property}: $px-values;
  #{$property}: $rem-values;
}

I only added support for % and auto values to this otherwise fantastic mixin.

Thanks for the @mixin. I've added a check so you don't end up with a double 0 value if something is set to 0.

// output the converted rules
    @if $px-values == $rem-values {
        #{$property}: $px-values;
    } @else {
        #{$property}: $px-values;
        #{$property}: $rem-values;
    }