Nobody weighs the smoke. A footprint is counted by measuring the fuel and multiplying by a factor. Picture a car with 40 litres of pure petrol in the tank. Each litre burned gives off about 2.3 kilograms of carbon dioxide, by the UK's 2025 figure. The tank is a sum waiting to be done.
A tonne of jet fuel1 tonne
The CO2 it makes3.16 tonnes
The factor is how much carbon dioxide one unit of a thing gives off. Jet fuel: about 3.16 tonnes for every tonne burned, the aviation body's figure; the UK's tables say 3.15. The smoke weighs more than the fuel, because each carbon atom takes two oxygen atoms from the air. Gas in a boiler: about 0.18 kg a kilowatt-hour.
40 litres
↓→
× 2.3
The tank holds 40 litres of pure petrol at about 2.3 kg of carbon dioxide each. How much carbon dioxide does the tank give off when it is all burned?
kg
Litres times kilograms a litre.
Right. About 92 kilograms. The petrol at a UK pump is nearer 2.1 a litre: about a tenth of it is made from plants, and the counting rules treat that share's carbon as zero. The sum is the same shape either way.
Train35 grams a passenger-km
Long-haul, economy69 grams a passenger-km
Domestic flight136 grams a passenger-km
Car, one person163 grams a passenger-km
A sum has two halves, and you can change either. Burn less: the quantity. Or burn something with a smaller factor. Per passenger and per kilometre, by the UK's 2025 factors: a train about 35 grams, a long-haul economy seat about 69 at the ground factor, a domestic flight about 136, a petrol car with one person about 163.
Move how 300 kilometres are travelled. The factors are the UK's 2025 figures. The flight shows two numbers: a plane's exhaust high up warms the air beyond its carbon dioxide, so the UK adds 70 in 100 to cover that.
At the ground factor10.6 kg of CO2 equivalent
Car, one person, for comparison48.8 kg of CO2 equivalent
How the 300 km are travelled: Train
Train: about 11 kg.
At the ground factor48.8 kg of CO2 equivalent
Train, for comparison10.6 kg of CO2 equivalent
How the 300 km are travelled: Petrol car, one person
Petrol car, one person: about 49 kg.
At the ground factor12.2 kg of CO2 equivalent
Train, for comparison10.6 kg of CO2 equivalent
How the 300 km are travelled: Petrol car, four people
Petrol car, four people: about 12 kg each.
At the ground factor40.7 kg of CO2 equivalent
With the high-altitude effect68.8 kg of CO2 equivalent
Train, for comparison10.6 kg of CO2 equivalent
How the 300 km are travelled: Domestic flight
Domestic flight: about 41 kg at the ground factor; about 69 kg with the high-altitude effect.
Which number decides whether the flight or the lone driver comes out worse?
The car is always worse. A plane spreads its fuel over many seats.
Not yet. A full car spreads its fuel over four seats too, and lands near the train. The lone car is the worst at the ground factor, not the car as such.
The flight is always worse. It is the biggest bar.
Not yet. At the ground factor the lone driver's 49 kg beats the flight's 41. The flight only comes out worse once the high-altitude effect is counted.
Whether the high-altitude effect is in the factor. At the ground factor the lone car is worse; with it, the flight is.
Right. Same plane, same fuel, two factors: 41 kg or 69 kg. The extra is not carbon dioxide; it is the water, soot and nitrogen oxides left high up, counted as if it were. The quantity did not move.
Move the control.
9,500 kWh of gas a year
↓→
× 0.18 kg
↓→
about 1.7 tonnes
Now a home. Ofgem's typical household burns about 9,500 kilowatt-hours of gas a year, mostly heating and hot water, by its figure from July 2026. At about 0.18 kg each, that is about 1.7 tonnes of carbon dioxide a year from one boiler. The reading is the gas meter, the factor is public, and the sum is anybody's to check.
10 tonnes of fuel
↓→
× 3.16
A plane burns 10 tonnes of jet fuel on a flight. The factor is 3.16. How many tonnes of carbon dioxide is that?
tonnes
Right. About 31.6 tonnes, more than three times the weight of the fuel. Heathrow's count of its flights is the same shape of sum: a fuel burn modelled for each flight from radar timings near the airport, the straight-line distance to the destination and the aircraft type, times the UK's factor.
Each card changes a footprint. Which half of the sum does it change: the quantity burned, or the factor?
One card at a time. Tap the pile it belongs to.
Card 1 of 6
Driving 2,000 kilometres less a year
Turning the heating down by two degrees
Replacing a gas boiler with a heat pump run on wind power
Running the trains on electricity from wind instead of diesel
Flying once a year instead of twice
Filling a plane with fuel made from waste, at a lower factor
Driving 2,000 kilometres less a year → The quantity
Not yet. The quantity. Fewer kilometres, less petrol. The car gives off the same amount for each litre it still burns.
Turning the heating down by two degrees → The quantity
Not yet. The quantity. Less gas goes through the meter. Each unit that does gives off the same 0.18 kg.
Replacing a gas boiler with a heat pump run on wind power → The factor
Not yet. The factor. The house is as warm, but each unit of warmth now comes from something with a far smaller factor than gas.
Running the trains on electricity from wind instead of diesel → The factor
Not yet. The factor. The same trains run the same miles; the smoke per mile falls because the fuel changed.
Flying once a year instead of twice → The quantity
Not yet. The quantity. Half the fuel burned on your behalf. The plane's factor is what it was.
Filling a plane with fuel made from waste, at a lower factor → The factor
Not yet. The factor. Same flight, same tonnes of fuel, less carbon counted for each tonne, because less of it came out of the ground.
3The quantity
vsvs
3The factor
Right. Three cards burn less; three burn something different. A footprint falls either way. Lesson 8's jet-fuel rule works on the factor; a bag charge works on the quantity.
up 30 in 100Kilometres
vsvs
down 20 in 100Carbon counted
A haulage firm says its lorries cut emissions by 20 in 100 this year. The same lorries drove 30 in 100 more kilometres. What must the factor have done?
Nothing. Driving more cannot cut emissions.
Not yet. It can, if the factor falls enough. The sum has two halves, and the firm is claiming the second half beat the first.
It fell by much more than 20 in 100. More kilometres and less carbon needs a far smaller factor.
Right. The quantity went up, so the factor had to do all of the work and then some. A fifth off the total with a third more driving is a factor cut by nearly 40 in 100.
It fell by 20 in 100 as well.
Not yet. A factor down 20 in 100 on kilometres up 30 in 100 is a total that rises. The claim needs the factor to fall faster than the driving grew.
Lesson complete
A footprint is a quantity times a factor. Burn less, or burn something with a smaller factor.