As a general rule, four inches is the optimal thickness for concrete sidewalks. The thickness of your sidewalk is determined by its purpose. A typical sidewalk is used for lightweight equipment such as skateboards, bikes, strollers, and scooters. Thus, it should provide a smooth solid surface to serve its purpose, however, if your sidewalk accommodates vehicles or it crosses a driveway then additional thickness is needed. Sidewalks that support motorized are eight inches thick and this helps to avoid buckling and cracks.