Cargo airlines sell aircraft space that is constrained by both weight and volume. A load of dense freight (e.g., boxes filled with lead) could exceed the air-craft's take-off weight limit long before the space in the aircraft is exhausted. Alternatively, low-density freight (e.g., boxes filled with feathers) would use up all the space in the aircraft before the aircraft's weight limit is reached. Airlines thus charge the higher of either the actual weight (measured by a scale) or the volume weight (calculated assuming an average density). A load master's task is to create the aero dynamically optimal load mix out of the total shipments booked onto a flight. The volume weight in periods calculated by dividing the volume of the shipment (in cubic inches) by 165.