What is a tonne-kilometre?

A tonne-kilometre is moving one tonne of cargo one kilometre. Freight emission factors are expressed per tonne-km, so multiplying weight in tonnes by distance in km and then by the factor gives the total CO2e for the shipment.

Why is airfreight so much worse than sea?

Aircraft burn enormous amounts of fuel to stay aloft, so per tonne-km they emit roughly 30 to 50 times more CO2 than a container ship. Sea freight is the lowest-carbon long-distance mode by a wide margin, which is why mode choice dominates freight emissions.

Where do the factors come from?

The per-tonne-km factors are DEFRA freight conversion factors: large container ships around 0.012 kg, rail around 0.027 kg, HGV around 0.11 kg, and long-haul airfreight around 0.6 kg CO2e per tonne-km. Real values vary with load factor and vessel size.

Does this cover well-to-wheel emissions?

These are operational (tank-to-wheel) factors for the transport itself. A full well-to-wheel or well-to-tank assessment adds the emissions of producing and distributing the fuel, which can increase the totals by roughly 15 to 25 percent depending on mode.

How does this fit Scope 3 reporting?

Upstream and downstream transport falls under Scope 3 Category 4 and Category 9 of the GHG Protocol. Activity-based tonne-km calculations like this are the recommended method when you know shipment weights and distances, giving more accuracy than spend-based estimates.

Shipping Emissions Calculator

This calculator estimates the carbon footprint of moving freight and lets you compare transport modes directly. You enter the cargo weight and distance, pick a mode, and it multiplies through DEFRA tonne-kilometre factors to give the CO2e, alongside a table of all four modes so the cleanest choice is obvious.

How it works

Freight emissions scale with how much you move and how far, multiplied by the mode’s emission factor per tonne-kilometre:

tonne_km = weight_tonnes × distance_km
co2e_kg  = tonne_km × factor_per_tonne_km

Typical DEFRA factors are about 0.012 kg CO2e per tonne-km for a large container ship, 0.027 for rail, 0.11 for an HGV, and 0.6 for long-haul airfreight — a roughly fiftyfold range that makes mode the single biggest lever in freight carbon.

Tips and example

Shipping 5 tonnes 8,000 km by sea produces around 480 kg CO2e, but the same load by airfreight emits roughly 24 tonnes — about fifty times more. Wherever lead times allow, shifting from air to sea or rail is the largest reduction available in freight. Improving load factor and avoiding part-empty trucks further lowers the effective per-tonne-km figure for road legs.