SASS item width calcs for Masonry grid__col-sizer grid__gutter-sizer

masonry-width-calc.scss

$gutter_size: 3%;
$number_of_columns: 3;
$total_gutter_space: ( $number_of_columns - 1 ) * $gutter_size;

.grid__item,
.grid__col-sizer {
  width: calc( ( 100% - #{$total_gutter_space} ) /  #{$number_of_columns} );
}

.grid__gutter-sizer { 
  width: $gutter_size;
}

.grid__item.double {
  width: calc( ( ( ( 100% - #{$total_gutter_space} ) / #{$number_of_columns} ) * 2 )  + #{$gutter_size} );
}