The choice between plane and ship is rarely settled by price per kilo alone. What counts is the value of the goods, the cost of holding stock, the risk of a stock-out and the nature of the cargo.
How air freight is charged
Air transport is charged on chargeable weight: the greater of actual weight and volumetric weight. Volumetric weight, in kilos, is calculated by multiplying length, width and height in centimetres and dividing by 6000, the IATA divisor, which corresponds to a density of 167 kg per cubic metre. Express courier operators usually use 5000.
Example: a 100 × 80 × 60 cm carton is 480,000 cm³, that is 80 volumetric kilos. If it weighs 50 kg, you pay for 80. The lighter and bulkier the cargo, the more the aircraft penalises it. Better packing, cutting the air inside boxes, pays off directly.
How sea freight is charged
In a full container (FCL) you pay for the container, whether or not you fill it. In consolidated cargo (LCL) you pay on the greater of tonnes and cubic metres, at one tonne to one cubic metre, plus consolidation, deconsolidation and terminal handling charges.
When air pays off
- High value per kilo: electronics, critical spare parts, pharmaceuticals.
- Short-life perishables, such as flowers and some fresh foods.
- Real urgency: a part that has stopped a production line or a site costs more per day than the freight differential.
- Samples and first orders testing a market.
When sea pays off
- Heavy, bulky or low-value-per-kilo cargo.
- Regular stock replenishment with advance planning.
- Dangerous goods in large volumes, which have very strict limits by air.
The cost that is not on the freight invoice
A ship takes weeks and a plane takes days, but what matters is the capital tied up during that time, the risk that demand changes in the meantime and stock-out losses. In Angola, where supply chains are long and foreign currency can be hard to obtain, predictability of lead time is often worth more than the lowest freight rate.
Middle options
Some combinations balance lead time and cost: sea-air, where cargo goes by sea to a hub and continues by air for the last leg, or splitting the order between an urgent part by air and the remainder by sea. Always ask the forwarder to compare the total delivered cost of each alternative, with lead times and risks.