Compare the carbon cost of two flights

Across the 238 flights we can measure, carbon per passenger-kilometre runs from 70 g to 100 g, with a median of 84 g. That is a spread of about 43 percent between the most and least efficient services we track, flying the same skies on the same day.

Three things account for nearly all of it. The first is the aircraft: a modern twin-engine widebody burns far less per seat than a four-engine design, which is why the 747 and A380 sit at the bottom of the efficiency table no matter who operates them. The second is cabin density: the same airframe fitted with more seats spreads the same fuel across more people, so a low-cost operator's A321 beats a long-haul carrier's on this measure while offering a great deal less room. The third is distance, and it cuts both ways - a longer sector carries the fuel that burns the fuel, but it also spends proportionally less of itself in the climb, which is the most fuel-hungry part of any flight.

What barely matters is the airline's name. Two carriers flying the same type on the same route land within a few grams of each other. The difference people imagine between a "green" airline and a conventional one is almost entirely a difference of fleet age and seat count.

Both sides use the same chain: measured fuel burn for the aircraft type at this sector length, multiplied by distance, multiplied by 3.16 to turn fuel into carbon dioxide, then divided across the passengers actually carried at an 83 percent load factor. Nothing here is modelled from first principles.

The per-passenger-kilometre figure is usually the more honest comparison. A long flight emits more in total simply by being long, so comparing a short hop with an intercontinental sector on totals alone tells you mostly about distance. Per passenger-km tells you about the aircraft and how densely it is configured.

Carbon dioxide only: contrails and nitrogen oxides at altitude roughly double aviation's warming effect, and the cabin split assumes a layout typical of each aircraft. The full method, sources and caveats are set out in how we calculate flight CO2.