A product distribution apparatus, shown in the form of an agricultural air seeder is disclosed having sensors to measure the mass flow through the distribution system, sensors to weigh the tank and the product therein, and sensors to measure the quantity of product in the tank. The sensors are used to measure the change in the quantity of product in the tank during a calibration process where the apparatus is operated over an area and the number of rotations of the meter are recorded. The data is then used to determine a mass flow rate per revolution of the meter. A monitoring method is also disclosed in which an ‘area to empty’ and ‘product needed’ to complete a field or task is displayed to the operator.