Road operators can use different criteria to determine the cost of their service: the weight transported, the volume transported, or the surface area occupied.
Our methodology
At Greenly, we use emission factors in kgCO2e/tonne.km. In the realm of carbon accounting, this "mass" approach (rather than a "volume" or "surface" approach) is the one most commonly used. This is because most road freight-related emission factors published in trusted databases are expressed in kgCO2e/tonne.km — for example the UK's GHG Conversion Factors for Company Reporting database or the French Environment Agency Base Carbone database. To stay consistent when computing GHG emissions, these emission factors and a "mass" approach are systematically used at Greenly.
How to calculate tonne-kilometers
1. Gather your data
Weight (tons): record the weight of the cargo in tons for each route.
Distance (kilometers): record the distance the cargo is transported in kilometers for each route.
2. Calculate ton-km for each route
For each route, use the formula: Ton-km = Weight (tons) × Distance (km). This gives you the ton-kilometers for each route.
3. Aggregate ton-km for all routes
Sum up the ton-km values of all routes to get the total ton-km. This total represents the combined transport effort across all routes in terms of tonnage and distance.
For example, suppose you have three transport routes:
Route 1: 10 tons × 100 km = 1, 000 ton-km
Route 2: 15 tons × 200 km = 3, 000 ton-km
Route 3: 20 tons × 150 km = 3, 000 ton-km
Total ton-km: 1, 000 + 3, 000 + 3, 000 = 7, 000 ton-km.
How emissions are computed
Once the ton-km are known, they are multiplied by the relevant emission factor (in kgCO2e/t.km). For example, if a 7.5 t lorry transports 1, 500 kg of goods over a distance of 100 km for one company (no other company uses the lorry), the corresponding GHG emissions are computed as follows:
GHG emissions = 0.378 kgCO2e/t.km (EF of the lorry) × 1.5 t × 100 km = 56.7 kgCO2e.
For more detail on how emission factors are applied, see How are GHG emissions computed from expenses and activity data?
Why does the weight matter?
With this method, heavy packages are penalized compared to lighter ones, since the emission factor is multiplied by the weight of the goods. It is frequently said that the fuel consumption of a lorry hardly depends on the weight transported, which is a genuine shortcoming of the methodology.
However, according to the UK Government, there is a real difference in fuel consumption between an empty lorry and a fully loaded one (see the UK Government GHG Conversion Factors for Company Reporting, available on the UK government website). In this database, the emission factors of 100% loaded lorries are between 17% and 65% higher than those of 0% loaded lorries — so the weight of the goods is not negligible.