A concrete floor should be about 4 inches thick if it is in a residential home or building. For a building supporting heavy loads, 5 to 6 inches is the recommended thickness. A good quality concrete floor will have even thickness throughout.
The ground needs to be dug into at an appropriate level to accommodate for slab thickness. This means that all ground materials, such as rocks and dirt, need to be dug away to at least 4 inches.
Although depth is important for concrete construction, it is also vital that the mix used can support 3,000 lbs. per square inch. Once the concrete is laid down, it needs to be properly insulated until it cures since it can either freeze or dry out.