Emissions
Carbon isn't close.
Money is arguable. Stress is my judgment. Carbon is arithmetic: how much fuel a plane burns per seat, and how dirty the electricity is where you plug the car in. It sits next to the verdict and never decides it — sometimes this site tells you to fly anyway.
Flying
11 kg per passenger for taxi, takeoff and climb, plus 0.2 kg per passenger-mile under 600 miles, 0.15 kg up to 2,300, and 0.14 kg beyond that. Short hops are worse per mile because climbing is where the fuel goes.
The total is then multiplied by 1.9. Contrails and high-altitude nitrogen oxides roughly double a flight's warming effect; the accepted range is 1.7 to 2.0. Take that multiplier out and flying still loses most trips, just by less.
Driving
Miles divided by 3.7 miles per kWh, times 1.1 because roughly a tenth of the energy never reaches the wheels, times the carbon intensity of the grid where the trip starts.
That figure is for the whole car. Four people in it means a quarter each — which is the part a flight can't match, since every extra passenger is another seat burning fuel.
Try it with your grid
| Trip | Driving | Flying | Difference | On the dirtiest grid |
|---|---|---|---|---|
| 200 miles | 0.8 kg | 171 kg | 221× worse to fly | 49 kg driving |
| 500 miles | 1.9 kg | 376 kg | 195× worse to fly | 123 kg driving |
| 800 miles | 3.1 kg | 441 kg | 143× worse to fly | 197 kg driving |
| 1,400 miles | 5.4 kg | 754 kg | 139× worse to fly | 345 kg driving |
The last column is West Virginia, the dirtiest grid on the continent at 830 g/kWh. Even there, 2 people in the car comes out ahead of the same seats on a plane at these distances. The US average is 370 g/kWh.
What this leaves out
- Building the car and the battery. That's a real number, but it's paid once and spread over the car's whole life, not this trip.
- Building the plane, and running the airport.
- How full the plane is. Airlines fly near capacity, so the per-seat figure is close to reality, but a half-empty flight is much worse per passenger than this shows.
- Time of day. Charging overnight on a grid running on wind is cleaner than the state average; charging on a hot afternoon is dirtier.
- The rides to and from the airport, and the hotel night a long drive forces.
If you don't own an electric car
A petrol car at 30 mpg puts out roughly 0.3 kg per mile — about 150 kg on a 500-mile trip, close to what one seat on that flight costs once contrails are counted. Split between two people it's still better than flying; alone it's roughly a draw. This site is about the electric case, and that's the honest comparison for everyone else.