You will find it on carbon labels and in company reports: a number followed by kg CO₂e. It looks technical. The idea behind it is simple, and once you have it, carbon figures become easier to read — and easier to compare with care.
CO₂e means carbon dioxide equivalent
Carbon dioxide is not the only gas that warms the planet. Methane and nitrous oxide do too — and gram for gram, they are far more powerful. A product’s footprint usually involves all three: CO₂ from burning fuel, methane from farming and waste, nitrous oxide from fertiliser.
Adding up grams of different gases would be meaningless, like adding euros to dollars without an exchange rate. So each gas is converted into the amount of CO₂ that would cause the same warming. That shared unit is carbon dioxide equivalent: CO₂e.
The exchange rate: global warming potential
The conversion factor is called global warming potential, and it is usually measured over 100 years. The values most carbon accounting relies on come from the Intergovernmental Panel on Climate Change. In its latest assessment:
| Gas | Warming compared with CO₂ over 100 years |
|---|---|
| Carbon dioxide (CO₂) | 1 |
| Methane (CH₄) from fossil sources | 29,8 |
| Methane (CH₄) from other sources | 27,0 |
| Nitrous oxide (N₂O) | 273 |
So one kilogram of nitrous oxide counts as 273 kg CO₂e, and one kilogram of methane leaking from a gas line counts as 29,8.
Fossil methane scores slightly higher because, when it breaks down, it leaves behind CO₂ that was not in the atmosphere before. And if methane’s number looks smaller than you have heard, that is because it does most of its warming in its first couple of decades. Measured over 20 years instead of 100, its effect is far larger. The 100-year figure is the convention, and it is the one you will almost always see.
How big is a kilogram of CO₂e?
Carbon is invisible, which makes the numbers hard to picture. A comparison can help, as long as it is read as a rough scale rather than an equivalence.
Illustrative example. The UK government’s 2025 greenhouse gas conversion factors put the exhaust emissions of an average petrol car at roughly one kilogram of CO₂e every six kilometres driven. A cotton shirt made in China, in a published life-cycle study, came to about 8,8 kg CO₂e over its whole life — about the same as a little over 50 km in that car. Both figures depend on their assumptions: the car’s average, and the study’s shirt, factories and use.
How to read a carbon figure on a product
Before comparing two figures, check what they cover and the quantity of product they describe. Three questions help:
- What it is per. Per item, per kilogram, per wear? Two figures only compare when they measure the same thing.
- What it covers. From raw material to the shop, or on through washing and disposal? Two honest numbers can differ simply because they count different stages.
- How it was arrived at. Every carbon figure is an estimate built on data and assumptions. Where a source explains its basis, that explanation helps you judge how far two figures can be compared.
A shared unit makes figures easier to read. It does not, on its own, make them comparable: that still depends on what each one covers and how it was worked out.
Karbonomics is developing ways to make product information more useful in everyday decisions. Read on: looking beyond shipping in a product’s carbon footprint.