Methodology

Food numbers are kilograms of CO₂-equivalent per kilogram of food, from Poore & Nemecek's 2018 study via Our World in Data. They're global averages across the whole chain: land, farm, feed, processing, transport, retail, packaging.

A second food round, “fridge and pantry”, draws on ADEME's AGRIBALYSE instead. It measures on a different basis, so the two never share a puzzle, and a cheese or a fish in one round won't match its number in the other. AGRIBALYSE also bakes in transport to a French shelf, which is the whole point of items like an air-freighted mango. Drinks there are measured per litre; for water-based drinks that's within about a percent of per kilo, so they sit on the same scale.

CO₂-equivalent rolls methane and nitrous oxide into one figure, so a kilo of beef and a kilo of apples sit on the same scale.

The “everyday choices”, “transport”, “energy” and “shock” rounds turn those factors into one real action — a 150 g burger, 100 km by train, a winter of gas heat, 100 burgers against one flight — using DEFRA's 2022/2024 factors for travel and home energy. Each item spells out the quantity it used, and that quantity is our editorial choice (why 100 burgers and not 50? to put it on a flight's scale). The underlying per-unit numbers are not.

Travel figures are per passenger. DEFRA's passenger-km factors assume a vehicle's average occupancy, so a full coach is lighter per person and a near-empty one heavier. Same trip, different sum depending on who else is aboard.

The “things” and “clothing” rounds count the cradle-to-gate cost of making one item, before you use it. Each value is cited from a life-cycle study: peer-reviewed papers, industry product reports (Apple, Trek, Levi's, Carbonfact) and government bag LCAs. A few, like a wool suit or a backpack, have weaker data and are marked as estimates. All are make-it-only; wearing and washing come later.

The “waste” round is the mirror image. It uses EPA's WARM model for the cost of sending one kilo of a material to landfill, counting only what happens after you bin it: methane as it rots, minus the carbon that stays locked in the ground. Making the item isn't counted, which is why a plastic bottle looks light here and a banana peel doesn't. WARM assumes US landfills, so read it as American rather than universal.

None of this is exact. Footprints swing with farm, region and method, so read the values as the shape of things, not a verdict to three decimals. Full citations live on the sources page.