Calculate a percent with SCSS/SASS

Another way:

$my_width: 4/12 * 100%;

div {
    width: $my_width; // 33.33333%
}

Sass will output the value in %.

Have you tried the percentage function ?

$my_width: percentage(4/12);
div{
    width: $my_width;
}

---

UPDATE

This function was updated since version 1.33.0 and now this is a correct method to do it:

@use "sass:math";

div {
    width: math.percentage(math.div(4,12));
}

Source: https://sass-lang.com/documentation/modules/math#percentage