Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.1)
// Compass (v0.13.alpha.10)
// ----

@mixin calc($property, $value, $fallback) {
  #{$property}: $fallback;
  #{$property}: -webkit-calc(#{$value});
  #{$property}: calc(#{$value});
}

body {
  @include calc(width, "100% - 20px", 98%);
}

SassMeister-output.css

body {
  width: 98%;
  width: -webkit-calc(100% - 20px);
  width: calc(100% - 20px);
}

Great mixin!