Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="box"></div>

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

// 8px is a reasonable smallest font-size
// 16px is a standard
//
// designing with 8px relative pixels

@function m8($value) {
  @return $value * 1em / 8;
};

.box {
  border: m8(1) solid black;
  padding: m8(4);
  height: m8(8);
  &:after {
    content:'';
    display: block;
    height: m8(8);
    background: red;
  }
}

SassMeister-output.css

.box {
  border: 0.125em solid black;
  padding: 0.5em;
  height: 1em;
}

.box:after {
  content: '';
  display: block;
  height: 1em;
  background: red;
}

SassMeister-rendered.html

<div class="box"></div>