Dynamic grid width calc for SASS.

grid.sass

$gutter: 20%

=columns($ac, $tc: 12, $gp: $gutter)
  // * $cw - column width
  // * $tc - total columns
  // * $ac - actual column (how many columns)
  // * $oc - one column (width)
  // * $tw - total width

  $tw: 100%
  $oc: $tw / $tc
  $cw: $oc * $ac

  // * $gp - gutter percentual (relative to 1 column width)
  // * $gt - gutter total

  $gt: $oc * $gp / $tw

  // * $fc - final column (width)

  $fc: $cw - $gt

  // * $gd - gutter distributtion

  $gd: $gt / $tc * $ac

  // * $rc - redistributted collumn

  $rc: $fc + $gd

  width: $rc

  &.left-offset
    margin-left: $gt

  &.right-offset
    margin-right: $gt

  &:first-child
    margin-left: 0

    &:last-child
      width: $cw