A dimensionally adjustable booster seat for use in combination with a seat belt system, including a seat back and a plurality or set of optional seating planes that are stack-able to form a 3-dimensional trapezoid base, and a coupling mechanism for detachably receiving combinations of the optional seating planes to the seat back. In the preferred embodiment, the seat back is contoured to blend with different combinations of the optional seating planes to provide a plurality of seating surface dimensions and heights, as to where a smaller passenger can be positioned within an adult seat belt system without modifying the natural path of the seat belt system.