Flight CO2 calculator
The fastest way to ruin a careful low-carbon year is one long-haul return flight. Here's how flying compares to the rest of your footprint, with DEFRA emission factors and the multiplier for high-altitude effects.
How we count it
DEFRA publishes per-passenger-km factors that include a 1.9× multiplier for non-CO2 effects at altitude (contrails, NOx). We follow that convention. If you want to count CO2 only, divide by 1.9.
- Short-haul (under 1,500 km): 0.158 kg CO2e/passenger-km
- Medium-haul (1,500–5,000 km): 0.149 kg CO2e/passenger-km
- Long-haul (over 5,000 km): 0.146 kg CO2e/passenger-km
Typical routes
Round-trip CO2 for popular routes. The Paris-aligned 2030 budget for a single person is 2.3 tCO2/year in total.
| Route | One-way km | Round-trip CO2e | % of Paris budget |
|---|---|---|---|
| London ↔ Paris | 344 | 0.11 tCO2e | 5% |
| London ↔ Rome | 1,432 | 0.45 tCO2e | 20% |
| Paris ↔ Athens | 2,095 | 0.62 tCO2e | 27% |
| London ↔ New York | 5,567 | 1.63 tCO2e | 71% |
| London ↔ Tokyo | 9,582 | 2.80 tCO2e | 122% |
| Sydney ↔ Singapore | 6,293 | 1.84 tCO2e | 80% |
| Los Angeles ↔ Tokyo | 8,800 | 2.57 tCO2e | 112% |
| Frankfurt ↔ Cape Town | 9,648 | 2.82 tCO2e | 122% |
Why one long-haul return is so much
A single London-NYC return is roughly 1.6 tCO2e per passenger. That's 70% of the entire Paris-aligned annual budget for one person — gone in two days. A London-Tokyo return runs ~2.8 tCO2e, more than the whole budget.
The math is simple: planes burn a lot of fuel per passenger-km, and distance compounds. There is no clean technological fix at scale on the 2030 timeline — sustainable aviation fuels are scarce, hydrogen is decades out, and electric aviation only works for short hops. The cheapest carbon you can save is the flight you don't take.
Train alternatives
Where high-speed rail exists, it slashes emissions. Same Paris-Athens distance by train (where feasible) is ~80 kg CO2 instead of 600+ kg by flight. London-Edinburgh by train is ~15 kg vs ~140 kg by flight. For Europe and East Asia, the rail option exists for most short and medium-haul; for transatlantic / transpacific, there is no alternative.
Sources
- UK DEFRA GHG Reporting Conversion Factors 2024 — per-passenger-km factors
- Our World in Data — Travel and CO2 — comparison to other modes of transport