rem

// Mixin that allows to specify arbitrary CSS properties with
	// unitless numbers. The output has rem unit with pixel fallback.
	// Shorthand assignments are supported too!
	$base_line: 10;

	@mixin rem($property, $values, $important:"") 
	{
		// Placeholder variables
		$shorthand_px:  "";
		$shorthand_rem: "";

		// Parameter $values might be a list of elements
		@each $value in $values
		{
			// Current value is a valid number and greater than 0
			@if $value != auto and $value != 0 and $value != inherit
			{
				// Add 'px' and 'rm' to the current value and store in
				// placeholder variables
				$shorthand_px: #{ $shorthand_px + " " + $value * $base_line + px };
				$shorthand_rem: #{ $shorthand_rem + " " + $value + rem };
			}
			// Current value is 'auto' or 0
			@else
			{
				// Add only 'auto' or 0 to the placeholder variables
				$shorthand_px: #{ $shorthand_px + " " + $value };
				$shorthand_rem: #{ $shorthand_rem + " " + $value };
			}
		}

		@if $important == !important
		{
			$shorthand_px: #{ $shorthand_px + " !important" };
			$shorthand_rem: #{ $shorthand_rem + " !important" };
		}

		// Output the CSS property in pixels and rems
		#{$property}:$shorthand_px;
		#{$property}:$shorthand_rem;
	}

http://intuio.at/en/blog/an-improved-sass-rem-mixin-for-unitless-numbers/