What is a carbon footprint? And what the number actually means
A carbon footprint is the total amount of greenhouse gas your activities release into the atmosphere over a period — usually a year — collapsed into a single number. That number is written in tonnes of CO₂-equivalent (t CO₂e), a unit that bundles carbon dioxide together with methane, nitrous oxide, and the other warming gases, each weighted by how much heat it traps. One figure, every gas, comparable across people and countries.
That last part is the point. A footprint is only useful if you can put it next to something.
What actually gets counted
Most of a personal footprint comes from four places:
- Getting around — car kilometres, flights, public transport.
- What you eat — especially red meat and dairy, which carry far higher emissions per kilo than plants.
- Heating and powering your home — gas, electricity, and how that electricity is generated where you live.
- The things you buy — manufacturing, shipping, and disposing of goods and services.
Each activity is multiplied by an emission factor — a published figure for how much CO₂e a kilometre, a kilowatt-hour, or a kilogram of beef produces. The factors we use come from DEFRA, the US EPA, and Our World in Data. Add the products up and you have a footprint.
It is an estimate, not a meter reading. Personal carbon accounting carries roughly ±20% uncertainty — good enough to tell you which decisions matter, not good enough to argue over the second decimal.
The numbers worth remembering
The global average is around 4 to 5 tonnes of CO₂ per person per year (Our World in Data). In wealthy countries it is often two to three times that — a typical North American footprint runs past 14 tonnes, a Western European one sits closer to 6–8.
The number that matters most is the target. A trajectory consistent with holding warming near 1.5 °C implies roughly 2 tonnes of CO₂e per person by 2030 (IPCC AR6). So for most people in rich countries, the honest framing isn’t “shrink a little” — it’s “the gap is large, and a few categories explain most of it.”
What moves the needle — and what doesn’t
This is where footprints earn their keep. The distribution is lopsided:
- A long-haul return flight can add 1–3 tonnes in a day — comparable to months of ordinary living.
- A petrol car driven 15,000 km a year is on the order of 2–3 tonnes.
- Diet swings by roughly a tonne between a heavy red-meat diet and a mostly-plant one.
- Home heating in a cold climate with gas is frequently the single biggest line.
Meanwhile, the things that feel virtuous — skipping a plastic straw, unplugging a phone charger, the occasional short shower — are real but rounding errors against the four above. Attention is a budget too. Spend it where the tonnes are.
Why we don’t end with “buy an offset”
It’s tempting to take the number, multiply by the price of a carbon credit, and call it neutral. We don’t, because most voluntary offsets do not deliver the reductions they claim, and “neutralising” a flight on paper doesn’t put the gas back. A footprint is information about your own decisions. The useful response is to change a decision, not to purchase absolution.
That’s the whole idea behind CO2Mate: log the activity, see the number, measure it against your country’s real per-capita average — and decide what’s worth changing. No greenwashing, nothing to sell you.
Sources: Our World in Data (per-capita CO₂); IPCC AR6 Working Group III (1.5 °C-aligned per-capita budgets); DEFRA and US EPA (emission factors).