Tonne-Kilometre (tkm)
Moving a pallet across a city and shipping a container across an ocean are wildly different jobs, yet a logistics report has to hold them on the same page. It needs a unit that captures both the weight of the goods and the distance they travel.
The tonne-kilometre is that unit — and it is what turns “how far did we ship, and how much did it cost the climate?” into a number.
A tonne-kilometre (tkm) is one tonne of freight moved one kilometre — the activity unit for freight-transport emissions. Total tkm = mass (tonnes) × distance. Emissions = tkm × an emission factor per tkm. It is the freight counterpart of the passenger-kilometre.
The tonne-kilometre is the workhorse activity unit of freight carbon accounting. It normalises consignments of different weights over different distances onto one scale, so a truck, a train and a container ship can be compared per unit of freight moved.
What a tonne-kilometre means
A tonne-kilometre (tkm) represents moving one tonne of goods over one kilometre. Like its passenger-side twin, the passenger-kilometre, it measures transport activity — the “amount of freighting” — not emissions in themselves. Two tonnes carried 50 km is 100 tkm, exactly the same activity as one tonne carried 100 km. Multiply that activity by an emission factor in kg CO₂e per tonne-km and you get the emissions.
Tonne-kilometre (tkm) — a unit of freight-transport activity equal to one tonne of goods transported one kilometre. Total tonne-kilometres equal the mass moved (in tonnes) multiplied by the distance travelled. Combined with a per-tkm emission factor, it yields the greenhouse-gas emissions of freight. The “tonne” is the metric tonne (1,000 kg) — not the US short ton (907 kg) or UK long ton (1,016 kg), so a US “ton-mile” is a different unit and must be converted before comparison.
The tonne-kilometre is the standard basis for Scope 3 Category 4 (upstream transport) and Category 9 (downstream transport) reporting, and the unit in which the freight standards express their emission factors.
From vehicle-km to tkm: payload and empty running
As with passenger transport, the tonne-kilometre is not the same as a vehicle-kilometre, and the factor that connects them is how heavily the vehicle is loaded:
Tonne-km = Vehicle-km × average payload (tonnes)
EF per tonne-km = EF per vehicle-km ÷ average payload
A vehicle’s emissions depend on how far it travels, not how much it carries. Spreading those emissions across a heavier payload lowers the per-tonne-km figure — which is why a fully loaded ship or train is so efficient per tkm, and a half-empty truck is not.
Two loading effects sit inside every published per-tkm factor. Load factor (payload utilisation) is the share of capacity actually used; a truck running at half its weight limit has roughly double the per-tkm intensity of a full one. Empty running — the return leg or backhaul with no load — adds vehicle-kilometres that carry zero tonne-kilometres, raising the effective intensity of the loaded legs. Modal per-tkm factors already embed an industry-average load factor and empty-running allowance, which is why a bespoke calculation from vehicle-kilometres and actual payload can differ from the default factor.
Emission factors per tonne-km, by mode
Because the tonne-kilometre is a common denominator, it exposes just how far apart the freight modes are. The current GLEC well-to-wheel factors per tonne-km span more than two orders of magnitude:
Emissions per tonne-km by mode. Basis: GLEC Framework v3.2, well-to-wheel per tonne-km. Air freight is roughly 150 times as carbon-intensive per tonne-km as a large bulk carrier — the single starkest spread in transport.
| Mode | kg CO₂e per tonne-km |
|---|---|
| Air freight (long-haul, average) | 0.788 |
| Road — articulated HGV (34–40 t) | 0.101 |
| Rail — diesel (mixed goods) | 0.0307 |
| Rail — electric (mixed goods) | 0.0108 |
| Sea — large bulk carrier (60k–100k dwt) | 0.0052 |
Live GLEC Framework factors, kg CO₂e per tonne-km, well-to-wheel basis. Sea intensity falls steeply with vessel size, so the band shown is a large-vessel example.
Two special cases: containers and air freight
Two parts of freight accounting depart from the plain tonne-kilometre and catch people out.
Container shipping uses TEU-kilometres, not tonne-kilometres. Deep-sea container factors are quoted per twenty-foot equivalent unit (TEU) moved one kilometre, because a container is booked and moved as a unit regardless of exactly how much it weighs. A TEU-km factor cannot be compared directly with a tonne-km factor without a tonnes-per-TEU assumption.
| Container ship (industry average) | kg CO₂e per TEU-km |
|---|---|
| Dry container | 0.0717 |
| Refrigerated (reefer) container | 0.1423 |
Live GLEC container factors — note the unit is per TEU-km, never per tonne-km. A refrigerated container carries roughly double the intensity of a dry one.
Air freight depends on the aircraft, not just the distance. Cargo travels either in a dedicated freighter or in the belly hold of a passenger flight, and the two are allocated differently. Counter-intuitively, belly freight is not always lower: on long-haul routes a dedicated freighter (0.608 kg CO₂e/tkm) beats belly hold (0.936), because a full freighter carries dense cargo more efficiently per tonne-km. Short-haul air freight is roughly twice long-haul, since take-off and climb dominate a short flight.
Formula and how to calculate
Working in tonne-kilometres is a two-step calculation:
Tonne-km = Mass (tonnes) × Distance (km)
Emissions = Tonne-km × EFtkm
Use the actual routed distance, not straight-line: road distances follow the network, and sea and air apply their own routing conventions. Mass is the goods’ weight; for containers, work in TEU-km with the container factor instead.
The Scope 3 Category 4 calculators apply these factors with the correct mode, vessel size and routing — the road freight, sea freight and air freight calculators each work in tonne-kilometres (or TEU-kilometres for containers), following the GLEC road-freight methodology.
Worked example
A 2-tonne consignment must travel 300 km. The tonne-kilometres are the same however it goes — but the emissions are not, because the mode sets the factor:
| Mode | EF (kg CO₂e/tkm) | Calculation | Emissions |
|---|---|---|---|
| Air freight (long-haul) | 0.788 | 600 × 0.788 | 472.8 kg CO₂e |
| Road — articulated HGV | 0.101 | 600 × 0.101 | 60.6 kg CO₂e |
| Rail — diesel | 0.0307 | 600 × 0.0307 | 18.42 kg CO₂e |
| Sea — large bulk carrier | 0.0052 | 600 × 0.0052 | 3.12 kg CO₂e |
The identical 600 tonne-kilometres of freight produces anywhere from 3.12 to 472.8 kg CO₂e — a roughly 150-fold difference set entirely by mode choice. This is why the tonne-kilometre is so valuable: it holds the freight task constant so the modes can be compared honestly.
Where tkm is used — and common mistakes
Tonne-kilometres are the activity unit for Scope 3 Categories 4 and 9, the basis on which the GLEC Framework and ISO 14083 quantify freight, and the denominator of a freight carbon-intensity metric (grams of CO₂e per tonne-km) used to benchmark carriers and set procurement policy.
A tonne-kilometre says how much freight moved how far — not how much carbon it cost. Mode and load factor decide that.
- Confusing tonne-km with vehicle-km. Emissions scale with vehicle-kilometres; converting to tonne-kilometres needs a payload assumption. A half-loaded truck emits per tonne-km roughly double a full one.
- Ignoring empty running. Backhaul legs carry zero tonne-kilometres but real vehicle-kilometres. Default factors embed an average allowance; a dedicated round trip with an empty return is worse than the default.
- Comparing tonne-km against TEU-km. Container shipping is quoted per TEU-km, not per tonne-km. Never place the two in the same column without a tonnes-per-TEU conversion.
- Mixing metric and non-metric tons. A tonne is 1,000 kg. US ton-miles use a 907 kg short ton — convert both mass and distance before comparing.
- Summing tonne-km with passenger-km. Freight and passenger activity are different units and must never be added into a single transport total.
Put the tonne-kilometre to work on a real shipment — across road, sea or air.
A tonne-kilometre (tkm) is one tonne of goods transported one kilometre. It measures freight-transport activity: total tonne-kilometres equal the mass moved in tonnes multiplied by the distance travelled. Multiplying tkm by an emission factor per tonne-km gives the freight emissions.
A vehicle-kilometre counts the distance the vehicle travels; a tonne-kilometre counts mass multiplied by distance. They are linked by payload: tonne-km = vehicle-km × average payload, and the factor per tonne-km equals the factor per vehicle-km divided by payload. Because emissions track vehicle-kilometres, how heavily the vehicle is loaded is what makes a mode efficient per tonne-km.
Multiply the tonne-kilometres by an emission factor for the mode, in kg CO₂e per tonne-km. For example, 600 tonne-km by articulated HGV at about 0.101 kg CO₂e/tkm is roughly 60.6 kg CO₂e. For container shipping, work in TEU-kilometres with the container factor instead.
Containers are booked and moved as whole units — a twenty-foot equivalent unit (TEU) — regardless of exactly how much cargo is inside, so deep-sea container factors are published per TEU-kilometre. To compare a TEU-km factor with a tonne-km factor you need an assumption for the average tonnes of cargo per TEU.
No. A tonne-kilometre uses the metric tonne (1,000 kg) and the kilometre. A US ton-mile uses the short ton (907 kg) and the statute mile (1.609 km). Both mass and distance differ, so a ton-mile figure must be converted before it can be compared with tonne-kilometres.
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