Sass function to output a property, along with it's values, with REM or PX units, depending on the value of $px-only

_functions.scss

$font-size: 16;
$px-only: true;

@function u($values){

    $list: ();

    @each $value in $values {

        @if $value == 'auto' {
            $list: append($list, $value);
        }

        @else {

            @if ($px-only) {
                $list: append($list, $value + px);
            }

            @else {
                $list: append($list, ($value/$font-size) + rem);
            }

        }

    }

    @return $list();

}

This:

$px-only: false;

.main {
    width: u(960);
    margin: u(0 auto);
}

$px-only: true;

.main {
    width: u(1120);
    margin: u(30 auto 30);
}

Generates this:

.main {
  width: 60rem;
  margin: 0rem auto;
}

.main {
  width: 1120px;
  margin: 30px auto 30;
}