How Thick Should a Concrete Slab Be?

Featured image with the headline How Thick Should a Concrete Slab Be, beside a pale rounded slab resting on a board over a layer of gravel, next to a wooden plank

How thick should a concrete slab be? See what the 2021 IRC, QUIKRETE and trade guides state for floors, walks, patios and driveways, and what they leave out.