rem() is a Sass mixin that converts pixel values to rem values.

rem.scss

/* 'rem' is a Sass mixin that converts pixel values to rem values
 * Returns 2 lines of code — regular pixel values and converted rem values 
 * 
 * Sample input:
 * .element { 
 * @include rem('padding',10px 0 2px 5px); }
 * 
 * Sample output:
 * .element {
 *   padding: 10px 0 2px 5px;
 *   padding: 1rem 0 0.2rem 0.5rem; }
 */

// Baseline, measured in pixels
// Value should be same as font-size value for the html element
// If html element's font-size set to 62.5% (of the browser's default 16px),
// then variable below would be 10px.
$baseline_px: 10px;

@mixin rem($property, $px_values) {
	// Convert the baseline into rems
	$baseline_rem: ($baseline_px / 1rem);
	// Print the first line in pixel values
	#{$property}: $px_values;
	// If only one (numeric) value, return the property/value line for it.
	@if type-of($px_values) == 'number' {
		#{$property}: $px_values / $baseline_rem;
	}
	// If more than one value, create list & perform equations on each value
	@else {
		// Create an empty list that we can dump values into
		$rem_values: ();
		@each $value in $px_values {
			// If the value is zero, return 0
			@if $value == 0 { 
				$rem_values: append($rem_values, $value); 
			}
			// If the value is not zero, convert it from px to rem
			@else {
				$rem_values: append($rem_values, ($value / $baseline_rem) );
			}
		}
		// Return the property and its list of converted values
		#{$property}: $rem_values;
	}
}