How to Calculate Your Carbon Footprint

Decide what you are counting before adding anything up, and whether it is per person or per household. If it is per person in a shared home, divide the household lines by the number of people (or another stated rule).

Each line is:

Emissions (kg) = Activity amount × Emission factor (kg per unit)

Units must match: kWh with kg/kWh, litres with kg/L, passenger-km with kg/passenger-km. Add the lines and divide by 1,000 for tonnes.

• CO₂ vs CO₂e — CO₂ counts carbon dioxide only. CO₂e also includes other greenhouse gases such as methane and nitrous oxide, converted to the amount of CO₂ with the same warming effect over a stated period. Use one basis for every line.
• Direct — emitted where you burn fuel: your boiler, your car's exhaust.
• Indirect — emitted elsewhere because of your use: at power stations for your electricity, in fuel supply chains, on farms for food, in factories for goods.

Companies classify emissions as Scope 1 (direct), Scope 2 (purchased energy) and Scope 3 (other indirect) under the GHG Protocol. The idea of direct and indirect helps personal accounting, but there is no single standard for personal footprints, so do not assume another calculator draws the lines the same way.

Three choices change a factor:

• Geography — electricity factors especially vary by country and region.
• Year — factor sets are revised annually as grids and fuel blends change.
• Lifecycle boundary — a fuel's combustion factor excludes extraction, refining and transport (well-to-tank); a lifecycle factor includes them. For flights, factors with and without a non-CO₂ (radiative forcing) uplift differ substantially.

Take every factor from one dataset and year where you can, such as the UK Government's greenhouse gas conversion factors or, in the US, EPA data. Note the version.

• Home electricity — annual kWh × a grid factor for your location and year. Choosing that factor (location vs market-based, losses, time of use) is covered in the electricity CO₂ guide.
• Heating fuels — convert to the factor's unit, usually kWh for gas. The gas to kWh converter handles meter volumes. Some datasets give separate gross and net calorific value factors; match how your kWh were calculated.
• Car travel — litres burned × a fuel factor; for an EV, the kWh drawn from the grid × the electricity factor.
• Flights — distance per leg × a passenger-km factor for the distance band and class. The transport carbon footprint guide covers distance, occupancy and radiative forcing.
• Food — quantities × lifecycle factors per food type; see the food carbon footprint guide.
• Goods and services — usually estimated from spending × a spend-based factor, or from product declarations. These are the most uncertain lines; label them as such.

Illustrative household activity, using one consistent factor set: UK Government conversion factors 2025, CO₂e. The activity amounts are invented; the factors are published values with the scope shown.

• EV charging at home is already in your electricity bill. Count it either in the electricity line or the car line, not both.
• Electric heating is part of electricity use; do not add it again as a heating line.
• Food and goods — spend-based estimates can overlap with item-by-item food estimates if you use both.
• Shared travel — if you count a shared car per vehicle, do not add each passenger's share again.

• Different boundaries (with or without food, goods, services)
• Different factor sets, years and geography
• Combustion-only vs lifecycle factors
• Flights with or without a radiative-forcing uplift
• Per person vs per household
• Activity data from bills vs estimates from questions about habits

None of these makes one calculator wrong; they answer slightly different questions. Compare totals only when the method matches.

Treat the total as an estimate with uncertainty: activity data from bills is fairly precise, factors are averages, and food and spending lines are much less precise. Rounding to two significant figures (5.3 t, not 5,281.7 kg) reflects that.

This guide does not label a footprint good, bad or average. Published national averages use their own boundaries and methods, which rarely match a personal calculation. The most reliable comparison is with yourself: the same method and factor set, year on year, and the breakdown by line, which shows where a change has most effect. Planning those changes is covered in the carbon reduction guide.

The Carbon Footprint Calculator covers home electricity, heating fuel, car fuel or EV charging, and flights — the energy and travel boundary of the worked example. Its fuel and flight factors are UK Government 2025 CO₂e factors and its electricity presets are the UK and US factors used by the Electricity CO₂ Calculator; every factor can be replaced. For a single trip, the flight vs train CO₂ tool compares modes. The methodology page documents factors and scope, and the Carbon Footprint hub collects all carbon guides.