columns($span: $columns)

$n: nth($span, 1)

$parent-n: parent-columns($span)
$expression: "(100% + #{$gutter}) * #{$n / $parent-n} - #{$gutter}"
$fallback: ($fallback-width - $gutter) * $n / $columns - $gutter
+calc(width, $expression, $fallback)

@each $breakpoint in $breakpoints
  +media($breakpoint)
    $parent-n: parent-columns($span)
    $expression: "(100% + #{$gutter}) * #{$n / $parent-n} - #{$gutter}"
    $fallback: ($fallback-width - $gutter) * $n / $columns - $gutter
    +calc(width, $expression, $fallback)

display: block
margin-right: $gutter
float: left