My favorite Sass function <3

_em.scss

// Converts pixels to EMs
// Usage: em(16) em(16px) em(16, 24) em(16px, 24px) em(16px, 1em)
@function strip-units($number) {
  @return $number / ($number * 0 + 1);
}

@function em($pixels, $context: 16) {
  $base: 16;
  $value: strip-units($pixels);
  $value_unit: unit($pixels);
  $c: strip-units($context);
  $c_unit: unit($context);

  @if $c_unit == "em" {
    @return $value/($base/$c)+0em;
  } @else {
    @return $value/$c+0em;
  }
}