Generated by SassMeister.com.

SassMeister-input.scss

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

@function get-px-from-vw($vw, $contextWidth) {
	@return (($contextWidth / 100) * $vw) + px;
}
@function strip-units($number) {
	@if type-of($number) == "number" and not unitless($number) {
		@return $number / ($number * 0 + 1);
	}

	@return $number;
}

@mixin font-size($sizeValue: 1.6, $line: $sizeValue + .5, $contextWidth:0) {
    //
    //  examples
    //  set as REM
    //  @include font-size(16.4, 14.5);
    //  
    //  set as VW
    //  @include font-size(6.45vw, 8vw);
    //
    @if type-of($sizeValue) == "number" and unitless($sizeValue) {
        font-size: ($sizeValue * 10) + px;
        line-height: ($line * 10) + px;
        font-size: $sizeValue + rem;
        line-height: $line + rem;
    } @else {
      @if $contextWidth > 0 and unit($sizeValue) == vw {
        font-size: get-px-from-vw(strip-units($sizeValue), strip-units($contextWidth));
        line-height: get-px-from-vw(strip-units($line), strip-units($contextWidth));
      }
      font-size: $sizeValue;
      line-height: ($line);
    }
}

.foo {
  @include font-size(6.25vw, 7vw, 320);
}
.bar {
  @include font-size(6.25px, 7vw, 320);
}
.baz {
  @include font-size(6.25vw, 7vw);
}

SassMeister-output.css

.foo {
  font-size: 20px;
  line-height: 22.4px;
  font-size: 6.25vw;
  line-height: 7vw;
}

.bar {
  font-size: 6.25px;
  line-height: 7vw;
}

.baz {
  font-size: 6.25vw;
  line-height: 7vw;
}