How I do a grid.

_grid.sass

// Variables
$total-columns   : 14
$grid-max-width  : 1400px
$max-page-width  : 1800px
$column-gutter   : 0%
$column-width    : (100 - $column-gutter * ($total-columns - 1)) / $total-columns
$span-real-width : round($grid-max-width * $column-width / 100%)

// Mixins
=space-span($space, $span: 1)
  width: percentage($space / ($span-real-width * $span))

=offset($cols)
  margin-left: $column-width * $cols + $column-gutter * $cols !important

=gutter($cols)
  margin-left: $column-gutter * $cols

=span($cols, $offset: false)
  float: left
  @if $cols == "1/2"
    width: 50%
  @else if $cols == "1/3"
    width: 33.333%
  @else if $cols == "1/4"
    width: 25%
  @else
    @if $offset
      margin-left: $column-gutter
    @else
      margin-left: 0

    width: $column-width * $cols + $column-gutter * ($cols - 1)

// Functions
@function col-width($cols : 1)
  @return percentage(1/$total-columns) * $cols
  
// Classes
@for $i from 1 through $total-columns
  .span-#{$i}
    +span($i)

  .offset-#{$i}
    +offset($i)

  .offset-#{$i}:not(:first-child)
    margin-left: $column-width * $i + $column-gutter * ($i + 1) !important

.span-#{$total-columns}
  margin-left: 0
  width: 100%