How this instrument works
Most food-carbon comparisons rank foods by weight — kg CO2e per kilogram of beef versus per kilogram of lentils — but that's a misleading yardstick, because a kilogram of beef and a kilogram of lentils don't deliver the same amount of usable protein. This calculator instead uses a per-100g-protein basis: it asks how much greenhouse gas it costs to deliver a fixed amount of the nutrient people actually eat meat and legumes for, which makes beef, chicken, tofu and eggs directly comparable on the metric that matters for a diet.
The factors come from Poore & Nemecek's 2018 Science meta-analysis, which pooled data from roughly 38,700 farms and 1,600 processors, packaging types and retailers across the global food system — the most comprehensive food life-cycle dataset assembled to date, distributed publicly via Our World in Data. We chose the per-protein framing deliberately: Our World in Data itself publishes several different per-kilogram-of-food presentations of this same underlying dataset, and those raw per-kg figures disagree with each other depending on which retail-weight or edible-portion assumptions are used. The per-100g-protein numbers are the more internally consistent basis across food types, so that's what this calculator uses.
Every number here is a global average, not a certificate for any specific product. Poore & Nemecek found impacts varying up to 50-fold between the highest- and lowest-impact producers of the very same food — a grass-fed beef operation on marginal land and a feedlot system both count as 'beef' in the average, and the low end of their reported beef range (9 kg CO2e per 100g protein) sits closer to some plant proteins than to the average (25 kg) used here. Treat this tool as a way to compare food categories, not to certify any specific meal.
- Select a food type — each option carries its own kg CO2e per 100 g of protein factor from the Poore & Nemecek dataset.
- Enter the grams of protein you want to account for. A whole chicken breast has roughly 50 g; a large egg has about 6 g.
- Read the carbon footprint in kg CO2e — the estimated greenhouse-gas cost of getting that much protein from that food.
- Switch the food type with the same protein amount entered to compare footprints for an equal nutritional trade.
Worked example — 200 g of protein from beef versus tofu
Say you're totaling up a week of beef-based dinners that together delivered 200 g of protein. Beef's factor is 25 kg CO2e per 100 g protein, so the footprint is 25 × (200 ÷ 100) = 25 × 2 = 50.0 kg CO2e — roughly the same emissions as driving an average gasoline car around 200 km.
Now swap in tofu for a smaller but directly comparable amount: 100 g of protein from tofu, at tofu's factor of 1.6 kg CO2e per 100 g protein, comes to 1.6 × (100 ÷ 100) = 1.6 kg CO2e. Even accounting for the fact that this second figure covers half the protein of the beef example, tofu's per-100g-protein footprint is roughly 1/15th of beef's — the core comparison this calculator is built to make honestly.
Questions
Why compare foods per 100g of protein instead of per kilogram of food?
Because a kilogram of beef and a kilogram of lentils deliver very different amounts of usable protein, so a per-weight comparison mixes up 'how much food' with 'how much nutrition.' Per-100g-protein puts every food on the same nutritional footing. This is also a deliberate methodology choice: Our World in Data publishes several per-kilogram-of-food presentations of the same underlying dataset that disagree with each other depending on retail-weight assumptions, while the per-protein basis is the more internally consistent way to compare across food types.
Where do these emissions factors come from?
Poore & Nemecek's 2018 study in Science, 'Reducing food's environmental impacts through producers and consumers,' which combined data from about 38,700 farms and 1,600 processors, packaging types and retailers worldwide — the largest food life-cycle assessment dataset compiled at the time. This calculator uses the per-100g-protein figures from that dataset as distributed through Our World in Data.
Why is beef's carbon footprint so much higher than chicken's or tofu's?
Cattle are ruminants: their digestion produces methane, a potent greenhouse gas, and beef production also typically requires far more land and feed per unit of protein than poultry, pork, eggs, or plant proteins. Chicken and pork convert feed to protein far more efficiently, and plant proteins like tofu and peas skip the feed-conversion step entirely, which is why their footprints land an order of magnitude lower.
Do these numbers reflect how a specific animal was raised, like grass-fed or organic?
No — these are global production averages, not certifications for any individual farm or product. Poore & Nemecek found up to a 50-fold spread in emissions between the highest- and lowest-impact producers of the same food; their reported beef range runs from about 9 to 105 kg CO2e per 100 g protein, versus the 25 kg average this calculator uses. Use this tool to compare food categories, not to audit a specific purchase.
How much protein is in common foods, to use with this calculator?
Roughly: a chicken breast (170 g) has about 50 g of protein, a large egg has about 6 g, a cup of cooked lentils has about 18 g, and 100 g of firm tofu has about 15 g. Check a nutrition label or the USDA FoodData Central database for a specific product's protein content before entering it here.