4-inch minimum for standard passenger vehicle traffic, 5–6 inches for areas with RV, truck, or heavy vehicle use. This is thicker than the 3.5-inch minimum some contractors use — Utah’s 100+ annual freeze-thaw cycles make thinner slabs significantly more susceptible to cracking and surface scaling, particularly when combined with de-icing salt use. Subbase preparation matters equally: 4–6 inches of compacted class II aggregate over a stable, non-expansive subgrade with a proper positive drainage grade away from the structure prevents the differential settlement that is the most common driveway failure mode on the Wasatch Front. RainFire Builders specifies a 4-inch minimum for residential driveways and 5–6 inches for heavy vehicle areas, on a properly compacted aggregate base with confirmed drainage grade before any forming begins.