Generated by SassMeister.com.

SassMeister-input-HTML.html

<p>This text should be 20px</p>
<div>This is unstyled text. If we set the body size to 62.5%, this would be 10px instead of 16px, which is fairly unusable.</div>

SassMeister-input.scss

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

//========= THE MIXIN =========//
@mixin font-size($sizeValue: 1.6) {
  font-size: ($sizeValue * 10) + px;
  font-size: ($sizeValue * 0.625) + rem;
}

//========= USAGE =========//
p { 
	@include font-size(2);
}

SassMeister-output.css

p {
  font-size: 20px;
  font-size: 1.25rem;
}

SassMeister-rendered.html

<p>This text should be 20px</p>
<div>This is unstyled text. If we set the body size to 62.5%, this would be 10px instead of 16px, which is fairly unusable.</div>