Online carbon calculators give wildly different answers for the same person. That's not because one is right and the rest are scams. They make different assumptions and rarely show them. Here's the maths, so you can judge any calculator, including the one I'm building for my tools page.
The core formula
emissions (kg CO₂e) = activity × emission factor
Add those up across everything you do. That's it. The same logic powers corporate carbon accounting.
The main categories
| Category | Activity data | Typical unit |
|---|---|---|
| Home electricity | Meter reading or bill | kWh |
| Home heating | Gas, oil, or heat pump use | kWh or litres |
| Car | Distance and fuel type, or fuel bought | km or litres |
| Flights | Distance and cabin class | passenger-km |
| Public transport | Distance by mode | passenger-km |
| Food | Diet type or spend | kg or € |
| Goods and services | Spend | € / $ / £ |
Emission factors: where the numbers come from
Reliable sources:
- UK government GHG conversion factors (DESNZ), updated annually
- US EPA emission factors hub
- National grid operators for electricity intensity
A worked example with illustrative numbers (not official factors):
| Activity | Amount | Factor (illustrative) | kg CO₂e |
|---|---|---|---|
| Electricity | 2,700 kWh | 0.2 kg/kWh | 540 |
| Car (petrol) | 8,000 km | 0.17 kg/km | 1,360 |
| Return flight, medium-haul | 3,000 km | 0.15 kg/pkm | 450 |
| Total (partial) | 2,350 |
Swap in official factors for your country and year.
Why calculators disagree
- Electricity factor: grid average vs your green tariff (location vs market-based).
- Flights: whether non-CO₂ effects (contrails etc.) are included via a multiplier. This can change flight emissions a lot.
- Food: diet categories vs detailed food data.
- Spend-based estimates: convert money to emissions with sector averages. They're crude but cover everything.
- Boundaries: some include goods, services, and public infrastructure; some don't.
Getting your own number
- Electricity and heating: get 12 months of bills. Use kWh, not cost.
- Car: odometer readings or fuel receipts.
- Flights: list them, calculate distances, note cabin class.
- Use official factors for your country.
- Estimate the rest with a spend-based calculator, and label it as an estimate.
To understand energy use first, try my Electricity Cost calculator, which works out kWh from appliance wattage and hours.
What to do with the number
The point isn't the number. It's finding the biggest lever. For most people in the US and Europe, that's usually heating, driving, or flying. See Heat Pump Running Costs and EV Charging Cost at Home.