Generated by SassMeister.com.

SassMeister-input-HTML.html

<div></div>
<div></div>

SassMeister-input.scss

// ----
// Sass (v3.4.0.rc.1)
// Compass (v1.0.0.alpha.20)
// ----

@mixin nthGrid($n, $marginX, $marginY) {
  float: left;
  width: ((100 - ($marginX * ($n - 1))) / $n);
  margin-right: $marginX;
  margin-bottom: $marginY;
  &:nth-child(#{$n+'n'} ) { margin-right: 0 }
  &:nth-child(#{$n+'n'} + 1) { clear: left; }
  border-bottom: 0;
}


.testSmall {
  @include nthGrid(2, 2%, 0%);
}

.testLarge {
  @include nthGrid(3, 2.15%, 2.15%);
}

SassMeister-output.css

.testSmall {
  float: left;
  width: 49%;
  margin-right: 2%;
  margin-bottom: 0%;
  border-bottom: 0;
}
.testSmall:nth-child(2n) {
  margin-right: 0;
}
.testSmall:nth-child(2n + 1) {
  clear: left;
}

.testLarge {
  float: left;
  width: 31.9%;
  margin-right: 2.15%;
  margin-bottom: 2.15%;
  border-bottom: 0;
}
.testLarge:nth-child(3n) {
  margin-right: 0;
}
.testLarge:nth-child(3n + 1) {
  clear: left;
}

SassMeister-rendered.html

<div></div>
<div></div>