Calculate a percent with SCSS/SASS

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