How the calculator works

Assumptions, in the order they apply

1. 1

   Servings

   A serving is the typical portion defined for each food in our comparison tool: 150 g of beef, lamb, pork or chicken; 140 g of fish; 250 ml of milk; 30 g of cheese; two eggs (100 g). Your real portions may be bigger or smaller.

2. 2

   Fish

   All fish and seafood are represented by farmed salmon, the best-documented species. Wild fish, prawns and tinned tuna have different footprints, some lower and some higher.

3. 3

   Country factors

   Where a country-specific figure exists for a food and metric, it is used. Otherwise the global average is used and the result is flagged 'global average used'. Country figures are usually available only for emissions and scarcity-weighted water.

4. 4

   Production systems

   The calculator uses the all-systems average for each food and country. Specific systems (grass-fed, dairy-herd beef, free-range) can be explored in the comparison tool but are not selectable here.

5. 5

   Replacement food

   Removed animal servings are assumed to be replaced by an even mix of lentils, tofu and chickpeas supplying the same protein. This is a simplification; many people replace meat with more of what they already eat.

6. 6

   Uncertainty

   Where a source reports a range for a factor, that range is used. Where it does not, we apply ±40% (0.6× to 1.4×), which is at the narrow end of the between-study spread for food life-cycle assessments. Ranges are added, not combined statistically, so they are conservative.

7. 7

   Savings range

   The saving is the difference between the middle estimates, with the baseline's relative uncertainty applied to it. This avoids the artefact of a large baseline range implying a change could increase impact.

8. 8

   Animals

   'Animals avoided' counts animals slaughtered per kilogram of meat or fish. Dairy and eggs count as zero because no animal is slaughtered per unit sold, although both systems involve animals. The figure is a statistical share, not a promise that a particular animal is spared.

9. 9

   Water

   Plain freshwater use is shown but is a poor guide to harm. Scarcity-weighted water use, where available, weights each litre by how stressed the source region is. Your selected country's water-stress score is shown for context but does not change the food factors, which depend on where the food was grown.

10. 10

    What is not counted

    Cooking, food waste, packaging differences between products, transport from shop to home, and health effects are not included. Dietary change also has effects on land that are not captured by area alone, such as whether freed land is restored.

11. 11

    Precision

    All outputs are rounded and prefixed 'about'. They describe an average person eating average products; your actual impact could differ by a factor of two or more.

The five changes

Each change keeps a fraction of your current servings of each food group. Removed servings are replaced by plant protein as described above.

Default diets by country

Weekly servings for a typical adult, used to prefill the calculator. Servings: beef a 150 g steak or portion of mince; lamb a 150 g portion; pork a 150 g portion; chicken a 150 g breast; farmed salmon a 140 g fillet; cow's milk a 250 ml glass; cheese a 30 g slice; eggs two medium eggs.

Every factor, per kilogram

The calculator uses the all-systems figure for each food. Country rows are used when that country is selected; otherwise the global row is used and flagged. Where no range is given by the source, 0.6× to 1.4× is applied.

Country water-stress scores

Shown for context next to water results. The score does not change the food factors, which depend on where the food was produced, not where it is eaten.

• Global average2.4 · Medium-high
• United Kingdom1.3 · Low-medium
• United States2.8 · Medium-high
• Germany1.8 · Low-medium
• India4.2 · Extremely high
• Brazil1.1 · Low-medium
• Australia3.1 · High

Everyday equivalents

• Driving: 0.17 kg CO₂e per km, an average petrol car.
• Tennis court: 261 m².
• Shower: 65 litres for eight minutes.

Sources

• Reducing food's environmental impacts through producers and consumers (Joseph Poore et al., 2018)
• FAOSTAT: Crops and livestock products — producing animals / slaughtered (Food and Agriculture Organization of the United Nations, 2024)
• National life-cycle assessment panel for food products (placeholder record) (Various national LCA studies, 2024)
• Aqueduct 4.0: Updated decision-relevant global water risk indicators (Samantha Kuzma et al., 2023)

See also how we rate evidence.