Tonne-kilometre (tkm)

A tonne-kilometre is one tonne of cargo carried one kilometre. It is the unit of work in freight, and every emissions standard — ISO 14083, the GLEC Framework, the UK government’s conversion factors — expresses a transport emission factor as grams of CO₂e per tonne-kilometre.

The reason it exists is that neither weight nor distance means anything alone. Ten tonnes over 100 km and one tonne over 1,000 km are the same amount of freight work, and to a first approximation the same amount of carbon on the same vehicle.

What it lets you compare #

Once emissions are expressed per tonne-kilometre, transport modes become directly comparable, and the differences are very large. A deep-sea container ship runs at roughly 10 to 25 grams; an articulated lorry at 60 to 200; long-haul air freight at 500 to 1,100. That gap is the whole reason distance is a poor guide to a shipment’s footprint — see food miles are misleading.

Getting the two numbers for your own shipment #

Both halves are harder than they look for a produce importer.

The tonnes are the cargo weight, not the gross weight of the box. Take the net weight off the packing list rather than the container’s laden weight, or you will be charging yourself for the tare of a reefer — which is heavier than a dry box, because of the insulation and the machinery.

The kilometres are the distance actually travelled on each leg, door to door. Not the straight line between two ports, and not the port-to-port figure on its own. A voyage through a transshipment hub is longer than the direct route, a diversion around a closed canal is much longer, and the road legs at both ends are usually where a produce chain’s transport carbon actually sits.

FAQ #

How do I calculate tonne-kilometres for a container shipment? #

Multiply the net cargo weight in tonnes by the distance travelled on each leg in kilometres, then add the legs together. A 22-tonne reefer load carried 11,000 km by sea and 400 km by road is 242,000 tonne-kilometres at sea and 8,800 on the road — and because the emission factors differ by roughly five times, that short road leg is not as small a share of the footprint as it looks.