Generated by SassMeister.com.

input.scss

.foo {
 @for $i from 1 through 24 {
    &--size-#{$i} {
      grid-column: calc(#{ 25 - $i} /2) / span #{$i};
      grid-row:  calc(#{25 - $i} / 2) / span #{$i};
    }
  }
}

output.css

.foo--size-1 {
  grid-column: calc(24 /2)/span 1;
  grid-row: calc(24 / 2)/span 1;
}
.foo--size-2 {
  grid-column: calc(23 /2)/span 2;
  grid-row: calc(23 / 2)/span 2;
}
.foo--size-3 {
  grid-column: calc(22 /2)/span 3;
  grid-row: calc(22 / 2)/span 3;
}
.foo--size-4 {
  grid-column: calc(21 /2)/span 4;
  grid-row: calc(21 / 2)/span 4;
}
.foo--size-5 {
  grid-column: calc(20 /2)/span 5;
  grid-row: calc(20 / 2)/span 5;
}
.foo--size-6 {
  grid-column: calc(19 /2)/span 6;
  grid-row: calc(19 / 2)/span 6;
}
.foo--size-7 {
  grid-column: calc(18 /2)/span 7;
  grid-row: calc(18 / 2)/span 7;
}
.foo--size-8 {
  grid-column: calc(17 /2)/span 8;
  grid-row: calc(17 / 2)/span 8;
}
.foo--size-9 {
  grid-column: calc(16 /2)/span 9;
  grid-row: calc(16 / 2)/span 9;
}
.foo--size-10 {
  grid-column: calc(15 /2)/span 10;
  grid-row: calc(15 / 2)/span 10;
}
.foo--size-11 {
  grid-column: calc(14 /2)/span 11;
  grid-row: calc(14 / 2)/span 11;
}
.foo--size-12 {
  grid-column: calc(13 /2)/span 12;
  grid-row: calc(13 / 2)/span 12;
}
.foo--size-13 {
  grid-column: calc(12 /2)/span 13;
  grid-row: calc(12 / 2)/span 13;
}
.foo--size-14 {
  grid-column: calc(11 /2)/span 14;
  grid-row: calc(11 / 2)/span 14;
}
.foo--size-15 {
  grid-column: calc(10 /2)/span 15;
  grid-row: calc(10 / 2)/span 15;
}
.foo--size-16 {
  grid-column: calc(9 /2)/span 16;
  grid-row: calc(9 / 2)/span 16;
}
.foo--size-17 {
  grid-column: calc(8 /2)/span 17;
  grid-row: calc(8 / 2)/span 17;
}
.foo--size-18 {
  grid-column: calc(7 /2)/span 18;
  grid-row: calc(7 / 2)/span 18;
}
.foo--size-19 {
  grid-column: calc(6 /2)/span 19;
  grid-row: calc(6 / 2)/span 19;
}
.foo--size-20 {
  grid-column: calc(5 /2)/span 20;
  grid-row: calc(5 / 2)/span 20;
}
.foo--size-21 {
  grid-column: calc(4 /2)/span 21;
  grid-row: calc(4 / 2)/span 21;
}
.foo--size-22 {
  grid-column: calc(3 /2)/span 22;
  grid-row: calc(3 / 2)/span 22;
}
.foo--size-23 {
  grid-column: calc(2 /2)/span 23;
  grid-row: calc(2 / 2)/span 23;
}
.foo--size-24 {
  grid-column: calc(1 /2)/span 24;
  grid-row: calc(1 / 2)/span 24;
}

settings.json

{
  "sass": {
    "compiler": "Ruby Sass/3.7.4",
    "extensions": {},
    "syntax": "SCSS",
    "outputStyle": "expanded"
  },
  "autoprefixer": false
}