Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="grid">
  <div class="grid__col"><div class="content">foo</div></div>
  <div class="grid__col"><div class="content">foo</div></div>
  <div class="grid__col"><div class="content">foo</div></div>
</div>

<div class="grid -gutters">
  <div class="grid__col"><div class="content">foo</div></div>
  <div class="grid__col"><div class="content">foo</div></div>
  <div class="grid__col"><div class="content">foo</div></div>
</div>

<div class="grid -gutters">
  <div class="grid__col"><div class="content">foo</div></div>
  <div class="grid__col -span-7"><div class="content">foo</div></div>
</div>

SassMeister-input.sass

// ----
// Sass (v3.4.9)
// Compass (v1.0.1)
// ----

*
  box-sizing: border-box

$gutter: 10px

.grid
  display: flex
  flex-wrap: wrap

  &.-gutters
    margin-left: -$gutter

    > .grid__col
      padding-left: $gutter

  &.-align-top
    align-items: flex-start

  &.-align-bottom
    align-items: flex-end

  &.-align-center
    align-items: center

.grid__col
  flex: 1

  @for $i from 1 through 10
    &.-span-#{$i}
      flex: none
      width: $i * 10%

  &.-align-right
    text-align: right


// Not part of grid, for demo only
.grid
  background: green
  padding-top: 10px
  padding-bottom: 10px
  margin-bottom: 5px
.grid__col
  background: red
  padding-top: 10px
  padding-bottom: 10px
.content
  background: blue
  

SassMeister-output.css

* {
  box-sizing: border-box;
}

.grid {
  display: flex;
  flex-wrap: wrap;
}
.grid.-gutters {
  margin-left: -10px;
}
.grid.-gutters > .grid__col {
  padding-left: 10px;
}
.grid.-align-top {
  align-items: flex-start;
}
.grid.-align-bottom {
  align-items: flex-end;
}
.grid.-align-center {
  align-items: center;
}

.grid__col {
  flex: 1;
}
.grid__col.-span-1 {
  flex: none;
  width: 10%;
}
.grid__col.-span-2 {
  flex: none;
  width: 20%;
}
.grid__col.-span-3 {
  flex: none;
  width: 30%;
}
.grid__col.-span-4 {
  flex: none;
  width: 40%;
}
.grid__col.-span-5 {
  flex: none;
  width: 50%;
}
.grid__col.-span-6 {
  flex: none;
  width: 60%;
}
.grid__col.-span-7 {
  flex: none;
  width: 70%;
}
.grid__col.-span-8 {
  flex: none;
  width: 80%;
}
.grid__col.-span-9 {
  flex: none;
  width: 90%;
}
.grid__col.-span-10 {
  flex: none;
  width: 100%;
}
.grid__col.-align-right {
  text-align: right;
}

.grid {
  background: green;
  padding-top: 10px;
  padding-bottom: 10px;
  margin-bottom: 5px;
}

.grid__col {
  background: red;
  padding-top: 10px;
  padding-bottom: 10px;
}

.content {
  background: blue;
}

SassMeister-rendered.html

<div class="grid">
  <div class="grid__col"><div class="content">foo</div></div>
  <div class="grid__col"><div class="content">foo</div></div>
  <div class="grid__col"><div class="content">foo</div></div>
</div>

<div class="grid -gutters">
  <div class="grid__col"><div class="content">foo</div></div>
  <div class="grid__col"><div class="content">foo</div></div>
  <div class="grid__col"><div class="content">foo</div></div>
</div>

<div class="grid -gutters">
  <div class="grid__col"><div class="content">foo</div></div>
  <div class="grid__col -span-7"><div class="content">foo</div></div>
</div>