gistfile1.txt

$cols: 12;
$gutter: 2%;
$one_col: (100% - ($gutter * ($cols - 1))) / $cols;

@mixin cols($num) {
	width: ($one_col * $num) + ($gutter * ($num - 1));
}

@mixin sub_cols($num_child, $num_parent) {
	$parent_size: ($one_col * $num_parent) + ($gutter * ($num_parent - 1));
	$child_size: ($one_col * $num_child) + ($gutter * ($num_child - 1));
	margin-left: ($gutter / $parent_size) * 100%;
	width: ($child_size / $parent_size) * 100%;
}

/* @group grid */
.row {
	#{enumerate('.span', 1, $cols, '')} {
		float: left;
		margin-left: $gutter;
	}

	@for $i from 1 through $cols {
		.span#{$i} {
			@include cols($i);
			
			@for $j from 1 through ($i - 1) {
				.span#{$j} {
					@include sub_cols($j, $i);
				}
			}
		}
	}
	
	> :first-child,
	.row > :first-child {
		margin-left: 0;
	}
	
}