A tonne-kilometre (often abbreviated tonne-km or tkm) is the standard unit for measuring freight transport activity. One tonne-km is one tonne of goods moved one kilometre. Move 5 tonnes of goods 100 km and you have 500 tonne-km of activity.
Why it matters
Freight emissions depend on both the weight of the load and the distance travelled, a heavier load over the same distance produces more emissions, and the same load over a longer distance produces more emissions. Tonne-km captures both in a single unit, making it the natural input for calculating transport emissions.
The DESNZ dataset publishes tonne-km emission factors for every common transport mode, articulated lorry, rigid HGV, van, rail freight, sea container, and air freight, in kgCO₂e per tonne-km. Multiplying activity (tonne-km) by the relevant mode-specific factor gives the emissions.
A practical example
A retailer ships 200 tonnes of goods an average of 350 km by articulated lorry over the year. That is 70,000 tonne-km. Multiplied by the DESNZ articulated-lorry factor (~0.08 kgCO₂e per tonne-km, depending on load profile), that gives roughly 5.6 tCO₂e, without ever needing fuel data from the haulier.
We use tonne-km calculations across upstream (Category 4) and downstream (Category 9) transport, sometimes building bespoke distance models from postcode geocoding for clients with high delivery volumes. See how on the methodology page.