calcGrid.sass

// Variables
$em: 1.5em

// Mixin
@mixin m-r
  margin-right: $em
@mixin m-r/2
  margin-right: calc($em / 2)
@mixin m-l
  margin-left: $em
@mixin m-l/2
  margin-left: calc($em / 2)

// Col Base
[class*="col-"] section
  padding: $em
  background: #ccc
  float: left

// Col Two
.col-2 section
  width: calc(100% / 2 - ($em / 2) )
  &:nth-of-type(1)
    @mixin m-r/2
  &:nth-of-type(2)
    @include m-l/2

// Col Three
.col-3 section
  width: calc(100% / 3 - ($em * 2 / 3) )
  &:nth-of-type(2)
    @include m-r
    @include m-l

// Col Four
.col-4 section
  width: calc(100% / 4 - ($em * 3 / 4) )
  &:nth-of-type(2)
    @include m-r/2
    @include m-l
  &:nth-of-type(3)
    @include m-r
    @include m-l/2

// Col Four
.col-5 section
  width: calc(100% / 5 - ($em * 4 / 5) )
  &:nth-of-type(2)
    @mixin m-r/2
    @include m-l
  &:nth-of-type(3)
    @mixin m-r/2
    @include m-l/2
  &:nth-of-type(4)
    @include m-r
    @include m-l/2