Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="container">
  <ul>
    <li>block 1</li>
    <li>block 2</li>
    <li>block 3</li>
    <li>block 4</li>
    <li>block 5</li>
    <li>block 6</li>
    <li>block 7</li>
    <li>block 8</li>
    <li>block 9</li>
    <li>block 10</li>
  </ul>
</div>

SassMeister-input.scss

// ----
// Sass (v3.4.12)
// Compass (v1.0.3)
// Bourbon (v4.2.0)
// Neat (v1.6.0)
// ----

@import "bourbon/bourbon";
@import "neat/neat";

.container {
  @include outer-container();
  
  background-color: gray;
  padding-right: 35px;
  
  ul {
    list-style-type: none;
  }
  
  li {
    @include span-columns(3);
    @include omega(4n);
    
    background: #ccc;
    height:50px;
    margin-bottom: 15px;
    text-align: center;
  }
}

SassMeister-output.css

* {
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
}

.container {
  *zoom: 1;
  max-width: 68em;
  margin-left: auto;
  margin-right: auto;
  background-color: gray;
  padding-right: 35px;
}
.container:before, .container:after {
  content: " ";
  display: table;
}
.container:after {
  clear: both;
}
.container ul {
  list-style-type: none;
}
.container li {
  float: left;
  display: block;
  margin-right: 2.35765%;
  width: 23.23176%;
  background: #ccc;
  height: 50px;
  margin-bottom: 15px;
  text-align: center;
}
.container li:last-child {
  margin-right: 0;
}
.container li:nth-child(4n) {
  margin-right: 0;
}
.container li:nth-child(4n+1) {
  clear: left;
}

SassMeister-rendered.html

<div class="container">
  <ul>
    <li>block 1</li>
    <li>block 2</li>
    <li>block 3</li>
    <li>block 4</li>
    <li>block 5</li>
    <li>block 6</li>
    <li>block 7</li>
    <li>block 8</li>
    <li>block 9</li>
    <li>block 10</li>
  </ul>
</div>