Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.12)
// Compass (v1.0.3)
// ----

$td-font-size-base: 16;
@function contains($list, $value) { @return if(index($list, $value), true, false); }
@function strip-unit($num) { @return $num / ($num * 0 + 1); }
@function is-unit-type($a, $unit) {
  $valid-units: 'deg', 'grad', 'turn', 'rad', 'px', 'em', 'rem', 'vh', 'vw',
                'vm', 'vmin', 'vmax', 'ex', 'ch';
  $validate-unit: contains($valid-units, $unit);
  @if $validate-unit { @return if(unit($a) == $unit, true, false); }
  @else              { @error '#{valid} is not a valid unit.'; }
}
@function convert-unit($value, $unit) {
  $units: ('px': 0px,
              'cm': 0cm,
              'mm': 0mm,
              '%': 0%,
              'ch': 0ch,
              'in': 0in,
              'em': 0em,
              'rem': 0rem,
              'pt': 0pt,
              'pc': 0pc,
              'ex': 0ex,
              'vw': 0vw,
              'vh': 0vh,
              'vmin': 0vmin,
              'vmax': 0vmax,
              'deg': 0deg,
              'turn': 0turn,
              'rad': 0rad,
              'grad': 0grad,
              's': 0s,
              'ms': 0ms,
              'Hz': 0Hz,
              'kHz': 0kHz,
              'dppx': 0dppx,
              'dpcm': 0dpcm, 'dpi': 0dpi,
          );
  @if map-has-key($units, $unit) { @return map-get($units, $unit) + $value; }
  @error 'Unknown unit `#{$unit}`.';
}

@function calc-rEm-from-px($px, $unit: 'rem', $return-px: false, $context: $td-font-size-base) {
  $valid-size-units: 'rem', 'em';
  $is-valid-unit:    contains($valid-size-units, $unit);
  $px-unitless:      strip-unit($px);
  $context-unitless: strip-unit($context);

  @if (not is-unit-type($px, 'px')) { $px: convert-unit($px, px); }
  @if $is-valid-unit {
    $return: convert-unit(($px-unitless / $context-unitless), $unit);

    @if ($return-px) { @return join($return, $px); }
    @else { @return $return; }
  } @else { @error 'Cannot convert #{$px} to #{$unit}'; }
}

@mixin r-em-px($property, $unit, $pxs.../*, $context*/) {
  $px-values:   ();
  $r-em-values: ();
  $pxs-count: length($pxs);
  $sole-px: $pxs-count == 1;

  $last: nth($pxs, -1);
  $context: if($sole-px, $td-font-size-base, $last);
  
  @if(not $sole-px) { $pxs-count: $pxs-count - 1; }

  @for $i from 1 through $pxs-count {
    $px: nth($pxs, $i);
    $rEM-then-px: calc-rEm-from-px($px, $unit, true, $context);

    $pxVal: nth($rEM-then-px, 2);
    $rmVal: nth($rEM-then-px, 1);

    @if $pxs-count > 1 {
      $pxVal: $pxVal + ' ';
      $rmVal: $rmVal + ' ';
    }

    $px-values:   append($px-values, $pxVal); // px
    $r-em-values: append($r-em-values, $rmVal); // rem or em
  }

  #{$property}: $px-values;
  #{$property}: $r-em-values;
}

p { @include r-em-px(margin, em, 4px, 14px); }
p { @include r-em-px(top, rem, 4); }

SassMeister-output.css

p {
  margin: 4px;
  margin: 0.28571em;
}

p {
  top: 4px;
  top: 0.25rem;
}