All international carriers base their shipping rates on the calculated Chargeable Weight. This is the greater of Actual Weight or Volumetric Weight.
Actual Weight is the weight of the package when put on a scale. Dimensional Weight (volumetric weight) is based on the size of the package. Large items that have a small Actual Weight, like pillows and lampshades will very often be charged according to their volumetric weight.
Volumetric weight is calculated using the following formulas:
Length (cm) x Width (cm) x Height (cm) / 5000.
E.g. 35cm x 24cm x 29cm / 5000 = 4.87Kg.
Length (in) x Width (in) x Height (in) / 139
E.g. 21in x 17in x 14in / 139 = 35.96lbs