a130/_sass/bourbon/functions/_px-to-em.scss

// Convert pixels to ems
// eg. for a relational value of 12px write em(12) when the parent is 16px
// if the parent is another value say 24px write em(12, 24)

@function em($pxval, $base: $em-base) {
  @if not unitless($pxval) {
      $pxval: strip-units($pxval);
  }
  @if not unitless($base) {
      $base: strip-units($base);
  }
  @return ($pxval / $base) * 1em;
}