Simple SCSS function to convert pixels to rems.

_functions.scss

/**
*
* Convert pixels to rems
* eg. for a relational value of 12px write rem(12)
* $base is usually the font-size on your body element.
* And it can be defined outside the function, in a variables.scss file for example.
*
**/

@function rem($pxval) {
  @if not unitless($pxval) {
      $pxval: strip-units($pxval);
  }

  $base: 16px;
  @if not unitless($base) {
      $base: strip-units($base);
  }
  @return ($pxval / $base) * 1rem;
}