Compare volume, not structural design
A thicker slab uses more concrete because volume increases in direct proportion to thickness when the footprint stays constant. The calculator shows a small comparison table for common example thicknesses and calculates the selected thickness as well.
Thickness requirements depend on use, loads, soil, reinforcement, base preparation, drainage, climate, and local codes. For a building, garage, driveway, or other structural application, consult the plans, applicable code, and a qualified local professional.
Example comparison
On a hypothetical 10 ft × 10 ft footprint, 4 in thickness corresponds to about 1.23 cu yd before waste; 6 in corresponds to about 1.85 cu yd. The comparison illustrates volume arithmetic only and is not construction guidance.