Calculators. Easy to use. The DPD combined dimension and volume weight calculators help you to find out quickly and easily... ...whether your parcel is within the maximum combined dimension: Maximum combined dimension National 400 cm   International 300 cm Formula for combined dimension: Length x twice the width x twice the height in cm ...whether parcels will be billed using the volume weight: For air carriage, parcels are charged by volume weight if this exceeds the actual weight. Volume weight tells us how much room a parcel takes up compared to its weight. Under the TACT rules laid down by the IATA (International Air Transport Association) volume weight is calculated as follows: Formula for volume weight: Length x width x height in cm / 6,000