The recommended thickness for a concrete sidewalk depends on the intended use and the local building codes or regulations. A thickness of 4 inches (10 centimeters) is usually sufficient for pedestrian sidewalks. However, a thicker concrete slab is necessary if the sidewalk will be subjected to heavier loads, such as for driveways or areas with frequent vehicle traffic. It’s best to consult with a professional concrete contractor or local authorities to determine the appropriate thickness based on your specific needs and local requirements.