A mixin that works out all the percentages for your grid. Just specify how many columns it's has and how big you want the gutter (in percentages) to be.

grid-mixin.scss

@mixin grid($columns, $gutter) {
  @if $columns > 1 {
		$fWidth: (1/$columns * 100%) - ($gutter * ($columns - 1) / $columns);
	
		@include clearfix;
		
		> li {
			width: #{$fWidth};
			box-sizing: border-box;
			float: left;
			margin-left: $gutter;
		
			&:nth-child(#{$columns}n + 1) {
				margin-left: 0;
				clear: both;
			}
		}
	}
}