Freight CO2 calculator: sea vs air for a garment shipment

Per tonne-kilometre, about 33 times as much, or about 56 times counting the extra warming effect of flying at altitude. Those are the UK Government’s 2026 greenhouse gas conversion factors: 0.5313 kg of CO2 equivalent per tonne-kilometre for an international freight flight, or 0.89939 kg with that effect, against 0.01612 kg for an average container ship. For one shipment the gap also depends on the two distances, because the sea route is usually longer than the flight. In the calculator’s opening example, 209 cartons of t-shirts weighing 1,442.1 kg, flying from Dhaka to London Heathrow emits about 6,141 kg CO2e, or 10,395 kg with the altitude effect, while shipping from Chattogram to Felixstowe around the Cape of Good Hope emits about 497 kg. Air is 12.4 times sea, or 20.9 times with the altitude effect. Through the Suez Canal the sea route is shorter, the sea figure falls to about 340 kg, and air is 18.0 times sea. These are estimates for comparing the two modes, not a measured footprint, but no realistic route closes a gap of that size.

Radiative forcing (RF) here means the extra warming caused by what aircraft emit at altitude besides carbon dioxide, such as water vapour, contrails and nitrogen oxides. The UK Government’s conversion factors publish two figures for freight flights: one for the direct emissions of carbon dioxide, methane and nitrous oxide only, 0.5313 kg CO2e per tonne-kilometre in 2026, and one that adds these indirect effects as a 70% increase on the carbon dioxide part, 0.89939 kg. The government’s guidance is that organisations should include the indirect effects to capture the full climate impact of air freight, and it says plainly that there is significant scientific uncertainty about their size. It treats aviation as the only form of transport where these effects are significant, so the sea factor has no equivalent. The calculator shows the air figure both ways, so you can see the range. If you report the result, say which figure you used, and use the same one every year so the trend means something.

Multiply the weight in tonnes by the distance in kilometres to get tonne-kilometres, then multiply by an emission factor for the mode, in kg CO2e per tonne-kilometre. In the calculator’s opening example, 1.4421 tonnes shipped 21,374 km from Chattogram to Felixstowe around the Cape is 30,823 tonne-kilometres, and at the UK Government’s 2026 factor of 0.01612 kg for an average container ship that is about 497 kg CO2e. Use the actual gross weight of the goods, not the air chargeable weight a freight bill uses. For a flight, use the great-circle distance between the airports: the government’s air factors already include an 8% uplift for indirect routes, stacking and circling, so do not add your own. The factors cover burning the fuel. Producing and delivering the fuel is listed separately as well-to-tank, 0.00365 kg per tonne-kilometre for a container ship and 0.13516 kg for a freight flight, and ISO 14083, the international standard for transport emissions, counts it in total emissions; with it included, the example comes to about 609 kg by sea and 7,703 kg by air. For a figure you publish, use your forwarder’s actual weights and routes, and ask whether it can report the shipment’s emissions from its own fuel data.

Sea distances come from Distances Between Ports (Pub. 151), published by the US National Geospatial-Intelligence Agency, eleventh edition, 2001, the edition the agency offers for download. It gives each port’s distance to junction points such as the Suez Canal, the Cape of Good Hope and Singapore, and a route is the sum of its sections. Chattogram to Felixstowe is 11,541 nautical miles (21,374 km) around the Cape and 7,903 (14,636 km) through Suez; to Sydney, through Singapore and the Torres Strait, 5,655 (10,473 km). These are mostly the shortest navigable routes, and real voyages are longer: a container first goes by feeder ship to a hub such as Colombo or Singapore, and a guide to ISO 14083 gives 15% as the common allowance between the shortest feasible and the actual sea distance. Flight distances are great-circle distances between the airports, worked out from the coordinates in the OurAirports open dataset: Dhaka to London Heathrow is 8,015 km. Air freight often changes planes at a hub; for that, choose another airport and type the sum of the legs. If your forwarder gives you the distance of your own route, type it in the same way.