Generated by SassMeister.com.

SassMeister-input-HTML.html

<a class="circle-button">foo</a>

SassMeister-input.scss

// ----
// Sass (v3.4.13)
// Compass (v1.0.3)
// ----

@function luminance($color){
    $rgba: red($color), green($color), blue($color);
    $rgba2: ();

    @for $i from 1 through 3 {
        $rgb: nth($rgba, $i);
        $rgb: $rgb / 255;
        $rgb: if($rgb < .03928, $rgb / 12.92, pow(($rgb + .055) / 1.055, 2.4));
        $rgba2: append($rgba2, $rgb);
    }

    @return (.2126 * nth($rgba2, 1) + .7152 * nth($rgba2, 2) + 0.0722 * nth($rgba2, 3))*100;
}

.foo {
  bar: luminance(red);
}

SassMeister-output.css

.foo {
  bar: 21.26;
}

SassMeister-rendered.html

<a class="circle-button">foo</a>