How to Customize Bootstrap Column Widths?

Answer 1

Bootstrap 4.1+ version of Antoni's answer:

The Bootstrap mixin is now @include make-col($size, $columns: $grid-columns)

.col-md-8half {
  @include make-col-ready();

  @include media-breakpoint-up(md) {
    @include make-col(8.5);
  }
}

.col-md-3half {
  @include make-col-ready();

  @include media-breakpoint-up(md) {
    @include make-col(3.5);
  }
}

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Source:

• Official documentation
• Bootstrap 4 Sass Mixins [Cheat sheet with examples]