Generated by SassMeister.com.

SassMeister-input-HTML.html

<div id="susyContainer">
  <div id="susyOffset">
    
  </div>
</div>

SassMeister-input.scss

// ----
// Sass (v3.3.8)
// Compass (v1.0.0.alpha.19)
// Susy (v2.1.2)
// ----

@import "susy";

#susyContainer {
  @include container(20rem);
  background: red;
  height: 40px;
  #susyOffset {
    @include span(4 at 3 of 12);   
    background: blue;
    height: 40px;
  }
}

SassMeister-output.css

#susyContainer {
  max-width: 20rem;
  margin-left: auto;
  margin-right: auto;
  background: red;
  height: 40px;
}
#susyContainer:after {
  content: " ";
  display: block;
  clear: both;
}
#susyContainer #susyOffset {
  width: 32.20339%;
  float: left;
  margin-right: 1.69492%;
  background: blue;
  height: 40px;
}

SassMeister-rendered.html

<div id="susyContainer">
  <div id="susyOffset">
    
  </div>
</div>