sass-site/source/assets/sass/_functions.scss

@function sl-px-to-rem($pixels,
                       $font-size: 16px) {
  @return ($pixels / $font-size * 1rem);
}

@function sl-px-to-em($pixels,
                      $font-size: 16px) {
  @return ($pixels / $font-size * 1em);
}

@function sl-px-to-unitless($pixels,
                            $font-size: 16px) {
  @return ($pixels / $font-size);
}