A footprint is a count
- 1 tonne of fuelBurns with oxygen3.16 tonnes of CO2
Picture one tonne of jet fuel, about 1,250 litres, going into a plane's wing. It burns. The carbon in it joins with oxygen from the air, and the gas that comes out of the engine weighs more than the fuel that went in. Burn one tonne of jet fuel and you get 3.16 tonnes of carbon dioxide.
- Every flight, van, boilerFuel burnedTimes 3.1619.4 million tonnes
A carbon footprint is the count of all of that, added up, for everything a place does in a year. The unit is tonnes of carbon dioxide. People say carbon for short. Heathrow counts its footprint every year and prints it. For 2025 the total was 19.4 million tonnes.
- Fuel burned10 tonnesCarbon dioxide made31.6 tonnes
A plane burns 10 tonnes of fuel on a flight. How many tonnes of carbon dioxide does that make? Use 3.16 per tonne.
tonnesTen times 3.16.
Right. 31.6 tonnes, more than three times the fuel's own weight, because most of the gas is oxygen pulled in from the air. The fuel tank was the small part. - 201921 million tonnes202318.8 million tonnes202419.4 million tonnes202519.4 million tonnes
Heathrow's footprint by year: 21.0 million tonnes in 2019, 18.8 in 2023, 19.4 in 2024, 19.4 in 2025. The 2019 number is the biggest. That was the busiest year before Covid, and newer planes have since cut fuel per seat. So 2025 carried more passengers than 2019 and made less carbon.
Tap the year with the biggest footprint.
Tap the part of the picture that answers it.
201921 million tonnes202318.8 million tonnes202419.4 million tonnes202519.4 million tonnes2019
Right. 21.0 million tonnes. The busiest pre-Covid year, flown on an older fleet. Heathrow's goal is that 2019 turns out to be its year of peak carbon, and it measures everything since against it.2023
Not yet. 18.8 million. Flights were still coming back after Covid, which is why it sits lowest of the four.2024
Not yet. 19.4 million. A record for passengers, but not for carbon, because the planes carrying them burned less per seat than in 2019.2025
Not yet. 19.4 million. Another passenger record and still below 2019. More people flew, on planes that burn less each.- 260 kgOne passenger, 2019231 kgOne passenger, 2025
A number that big is hard to feel. Divide it by passengers and it becomes 231 kilograms of carbon per passenger in 2025, down from 260 in 2019. That is Heathrow's own figure. It counts the whole flight out, not just the airport's share of it. The next lesson is about who counts what.
Slide to your guess: how much of Heathrow's 19.4 million tonnes comes from the planes, rather than from the airport's own buildings, vans and the traffic around it? In 100.
Slide the marker to where you think the answer is, then press Lock. Nothing is scored.
your guess · 0 in 100the answer · 96 in 100- 1Half50 in 100
- 2All of it100 in 100
Your guess: —
Almost all of it. Ninety-six in every hundred tonnes is aircraft, in the air and on the runway. The terminals, the vans, the supply chain and every car and train journey to the airport together make the other four. Guesses usually land lower, because the buildings are the part you can see.- In the air18.6 million tonnes · 96%
- On the ground0.86 million tonnes · 4%
So the footprint is really two counts. The big one is the planes: 18.6 million tonnes in 2025. The small one is everything on the ground: 0.86 million tonnes. Heathrow calls them in the air and on the ground, and sets a separate target for each. The ground is small, but it is the part the airport runs itself.
- 50 flights a day2,000 tonnes a year?
A small regional airport with 50 flights a day publishes a footprint of 2,000 tonnes a year. A planespotter says that is far too small. Who is probably right?
The planespotter. Fifty flights a day burn far more than that. The airport has likely counted only its own buildings and vehicles.
Right. Fifty flights a day is 18,000 a year. Even short flights burn tonnes of fuel each. A count that small has left the planes out, which is the choice the next lesson is about.The airport. A footprint is only what the airport itself burns.
Not yet. That is one way to count, but then the number says nothing about the planes, which are nearly all of the carbon. Heathrow counts the flights; this airport did not say what it counted.Neither. A footprint cannot be checked.
Not yet. It can. Fuel sold at the airport is known to the tonne, and 3.16 turns it into carbon. A footprint is a count, and counts can be checked.
Lesson complete
A carbon footprint is a count in tonnes: fuel burned, times what it turns into.