Every carbon footprint, from a household estimate to a company’s annual report, is built the same way. You list what you do, find a published factor for each activity, multiply, and add up. This page shows the method with real factors, so you can check any calculator’s result or build your own estimate.
It is part of our carbon footprint section. If you just want a number, the carbon footprint calculator does these sums for you in four steps.
The basic formula
The GHG Protocol, the accounting standard behind most corporate reporting, says the most common approach is to apply documented emission factors: ratios that relate greenhouse gas emissions to a measure of activitySource 1. In plain terms:
Emissions (kg CO₂e) = activity data × emission factor
- Activity data is what you did: kWh of electricity, litres of fuel, kilometres travelled, pounds spent.
- The emission factor is the CO₂e released per unit of that activity, taken from a published dataset.
- CO₂e puts every greenhouse gas on one scale, so methane and nitrous oxide count alongside carbon dioxide.
Add the results for every source and divide by 1,000 to get tonnes.
Five steps to a footprint
The GHG Protocol sets out five steps for companies, and they work just as well for a householdSource 1:
Step 1: Identify your emission sources
List what burns fuel or uses energy, and what you buy: home energy, vehicles, flights, food, goods and services.
Step 2: Choose a calculation approach
Decide whether to use physical units (kWh, litres, km) or spending for each source.
Step 3: Collect activity data and choose emission factors
Pull a year of bills, mileage and receipts, and match each to a factor from one consistent dataset.
Step 4: Apply the factors
Multiply each activity by its factor, in a spreadsheet or a calculator.
Step 5: Add it up
Sum the results into one yearly total, and keep the breakdown so you can see the biggest sources.
The standard also asks for five principles: relevance, completeness, consistency, transparency and accuracySource 1. For a household that means: count everything that matters, use the same method each year, and write down which factors you used.
Worked examples with real factors
The UK government publishes a free set of conversion factors every year, maintained by the Department for Energy Security and Net Zero (DESNZ)Source 3. These examples use its 2026 factors.
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| Activity | Amount | Factor (kg CO₂e per unit) | Result (kg CO₂e) |
|---|---|---|---|
| UK grid electricity, incl. transmission losses | 1,000 kWh | 0.144 | 144 |
| Natural gas | 1,000 kWh | 0.182 | 182 |
| Average petrol car | 1,000 km | 0.162 | 162 |
| National rail | 1,000 passenger-km | 0.031 | 31 |
As of Oct 2026 · Source: [3] DESNZ greenhouse gas conversion factors 2026
Two details trip people up. First, a car factor is per vehicle-kilometre, so share it between the people in the car; rail and bus factors are already per passengerSource 3. Second, the 0.182 gas factor is per kWh on a gross calorific value basisSource 3, so check that your bill uses the same basis.
Flights, food and shopping
Some sources need more than one step.
- Flights. Work out the distance, pick the factor for the type of flight and cabin, and decide whether to include non-CO₂ effects at altitude. DESNZ publishes separate factors with and without these effects, recommends a multiplier of 1.7 as a central estimate, and builds an 8% uplift for indirect routes into its flight factorsSource 4. Our flight emissions calculator applies these rules for any route.
- Food. Few people know how many kilograms of each food they eat in a year, so diet-based figures are easier. A study of 55,504 UK adults found median diet emissions of 2.47 kg CO₂e a day for vegans and 7.04 kg for medium meat-eatersSource 5. Multiply by 365 for a yearly figure: about 0.9 and 2.6 tonnes.
- Shopping. Use spend-based factors, which estimate the full supply-chain emissions per pound spent. Defra’s 2023 multipliers are about 0.35 kg CO₂e per pound spent on clothing and about 1.8 kg per pound on phonesSource 6.
Activity data or spending?
The Scope 3 Standard splits data into two kindsSource 2:
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| Data type | What it is | When to use it |
|---|---|---|
| Primary data | Data from suppliers or partners about the specific activity, such as a supplier’s own emissions figure | For your largest sources, when you can get it |
| Secondary data | Industry averages, government statistics, published databases, financial data and proxies | To fill gaps and to screen for your biggest sources |
For a household, physical units from bills are the primary data that matter most. For everything you only know the price of, spend-based factors are a fair first estimate.
Common mistakes
- Mixing datasets. Factors from different years or countries are built on different assumptions. Use one set throughout, and note the year.
- Using another country’s grid. Electricity factors vary widely by country. Outside the UK, use your national grid intensity; Our World in Data publishes figures from Ember for most countriesSource 7.
- Counting the household as one person. Divide shared home energy by the number of people who live there; travel, food and shopping are counted per person.
- Double counting. Don’t add a spend-based figure for fuel you have already counted in litres.
- Treating the result as exact. Average factors can’t describe your exact boiler, car or diet. The number is an estimate that points to your biggest sources.
Recalculate once a year
Run the same calculation each year with the latest factors and the same method. The GHG Protocol asks companies to keep methods consistent so that comparisons over time mean something, and the same habit makes your own progress visibleSource 1.
For companies: scopes and a base year
A company footprint uses the same formula, organised into three scopes. Scope 1 is fuel the company burns, scope 2 is purchased electricity and scope 3 is the rest of the value chain; scope 1, 2 and 3 explained covers each. The GHG Protocol notes that most small and medium-sized companies calculate scope 1 from purchased fuel and scope 2 from metered electricity, both with published factorsSource 1.
Companies also choose a base year with verifiable data, and set a policy for recalculating it after acquisitions, divestments or method changesSource 1. Our business emissions section shows how to turn that baseline into a reduction plan.
What to do with the number
A footprint is only useful if it changes what you do. Compare it with the average carbon footprint by country, then go to how to reduce your carbon footprint and start with the largest line in your breakdown. Credits come last, for what you can’t yet cut.