This National Ventilation MONV619 Monsoon Circular Adapter Reducer is a white PVC duct adapter perfect for fan vent installations. It is used to decrease or increase the diameters of a duct system, allowing the connection of two different-sized flexible or semi-rigid ducting with clamps and duct tape. The adapter measures 150mm (6 inch) to 100mm (4 inch) with an outside diameter of 155mm and inside diameter of 150mm at one end, transitioning to 101mm outside and 98mm inside diameter at the other end. Its depth is 60mm.