tonkilometer

A tonkilometer, commonly written as tonne-kilometre or tkm, is a unit of transport work representing the movement of one tonne (1000 kilograms) over a distance of one kilometre. It is the product of mass and distance, and is used to quantify the amount of freight or passenger movement rather than energy or emissions on its own.

Calculation is straightforward: multiply the transported mass in tonnes by the distance in kilometres. For example, shipping 20 tonnes over 150 kilometres equals 20 × 150 = 3,000 tonne-kilometres. If a vessel carries 500 tonnes for 2,000 kilometres, the result is 1,000,000 tonne-kilometres.

Tonnes-kilometres are widely used in transport analysis and logistics to express activity level and to normalize comparisons across modes (road, rail, sea, air) or routes. They are also used in evaluating energy efficiency and emissions intensity, for instance by reporting emissions per tonne-kilometre to compare how different transport modes perform relative to the amount of cargo moved.

Limitations should be noted: tonne-kilometres measure transport work, not energy consumption or emissions directly. They do not account for differences in vehicle efficiency, load factors, or idle distances. Therefore, while tkm is useful for activity and normalization, it is often complemented by energy and emissions metrics for a complete environmental assessment.

See also: tonne, kilometre, tonne-kilometre as a unit of transport activity, emissions per tonne-kilometre.