CO₂e, short for carbon dioxide equivalent, is a unit that expresses the warming effect of any greenhouse gas as the amount of carbon dioxide that would cause the same warming. It lets you add different gases, such as methane and nitrous oxide, into one number. Every carbon footprint, emission factor and climate target you will come across is stated in it.
This guide explains how the conversion works, why the same activity can produce slightly different numbers in different reports, and how to read the unit correctly.
Why CO₂ alone is not enough
Carbon dioxide is the most common greenhouse gas from human activity, but it is not the only one. Methane leaks from gas systems, livestock and landfill. Nitrous oxide comes from fertiliser and combustion. Refrigerant gases escape from air conditioning and fridges. Each traps a different amount of heat and stays in the atmosphere for a different length of time.
Reporting each gas separately would make footprints impossible to compare. Is a farm that emits 10 tonnes of methane better or worse than a factory that emits 200 tonnes of carbon dioxide? CO₂e answers that by converting everything to one scale.
How the conversion works
Each gas has a global warming potential (GWP). It measures how much heat one kilogram of the gas traps over a set period, compared with one kilogram of carbon dioxide over the same period. Carbon dioxide is the reference, so its GWP is always 1.
The formula is: mass of gas × GWP = CO₂e.
| Gas | 100-year GWP (IPCC Sixth Assessment Report) | Common source |
|---|---|---|
| Carbon dioxide (CO₂) | 1 | Burning fuel |
| Methane (CH₄), fossil | 29.8 | Gas leaks, coal mining |
| Methane (CH₄), non-fossil | 27.0 | Livestock, landfill |
| Nitrous oxide (N₂O) | 273 | Fertiliser, combustion |
| HFC-134a (a refrigerant) | 1,530 | Vehicle and building air conditioning |
Some industrial gases, such as sulphur hexafluoride, have a GWP above 23,000, so even small leaks matter.
A worked example
A site emits 2,000 kg of carbon dioxide and 10 kg of fossil methane in a year.
- Carbon dioxide: 2,000 × 1 = 2,000 kg CO₂e
- Methane: 10 × 29.8 = 298 kg CO₂e
- Total: 2,298 kg CO₂e, or about 2.3 tonnes
Ten kilograms of methane adds almost 15% to the total. Now add a refrigerant leak of just 1 kg of HFC-134a: 1 × 1,530 = 1,530 kg CO₂e. One kilogram of that gas is worth more than three quarters of the site’s entire carbon dioxide output. This is why small quantities of potent gases cannot be ignored.
Want the number without the spreadsheet? Enter four figures from your bills and get your monthly CO₂e split by scope. Open the free Business Carbon Calculator
Why the time period matters
Gases do not last equally long. Methane breaks down in roughly 12 years. Nitrous oxide lasts more than a century. A share of emitted carbon dioxide stays in the atmosphere for centuries or longer.
Because of this, a gas’s GWP depends on the period you measure over. The standard for reporting is 100 years, written GWP100. Over 20 years, fossil methane has a GWP of about 82.5 instead of 29.8, because nearly all of its warming happens early. Both figures describe the same gas. The 100-year value is the convention for company footprints and national inventories. The 20-year value is used when the focus is on warming over the next couple of decades.
Unless a report says otherwise, assume CO₂e means GWP100.
CO₂ vs CO₂e: what is the difference?
CO₂ counts one gas. CO₂e counts all greenhouse gases, converted to a common unit. When a report says a company emitted 50 tonnes of CO₂e, that figure may include methane, nitrous oxide and refrigerants as well as carbon dioxide.
For fuel and electricity the two numbers are close, because carbon dioxide makes up nearly all of the total. For agriculture, waste and refrigeration they can be far apart, because methane, nitrous oxide and refrigerants dominate.
CO₂e vs carbon: a common mix-up
Some older sources report tonnes of carbon, not tonnes of carbon dioxide. They are not the same. A carbon dioxide molecule is heavier than the carbon atom inside it, so one tonne of carbon corresponds to about 3.67 tonnes of carbon dioxide. If a figure looks roughly four times too small, check which unit it uses.
Why published numbers differ slightly
GWP values are updated as climate science improves. The Intergovernmental Panel on Climate Change publishes a new set with each assessment report.
| Gas | Fourth report (2007) | Fifth report (2013) | Sixth report (2021) |
|---|---|---|---|
| Methane | 25 | 28 | 27.0 to 29.8 |
| Nitrous oxide | 298 | 265 | 273 |
| HFC-134a | 1,430 | 1,300 | 1,530 |
Reporting rules name which set to use, and they do not all use the latest. As a result, two footprints of the same activity can differ by a few percent. This is not an error. State which values you used, keep them the same from year to year, and note it if you switch.
Which gases are included
The GHG Protocol covers seven gases or groups of gases, the same ones covered by international climate agreements:
- Carbon dioxide (CO₂)
- Methane (CH₄)
- Nitrous oxide (N₂O)
- Hydrofluorocarbons (HFCs)
- Perfluorocarbons (PFCs)
- Sulphur hexafluoride (SF₆)
- Nitrogen trifluoride (NF₃)
Carbon dioxide released by burning biomass, such as wood pellets or biofuel, is handled differently. Under the GHG Protocol it is reported separately from the three scopes, because the carbon was recently absorbed from the atmosphere by the plants.
Units you will see
- kg CO₂e: kilograms, used for single activities and emission factors.
- t CO₂e: tonnes, used for company and product footprints. One tonne is 1,000 kg.
- kt and Mt CO₂e: thousand and million tonnes, used for large companies, sectors and countries.
- g CO₂e per kWh or per km: grams, used for intensity figures such as grid electricity or vehicle emissions.
Do you need to apply GWP yourself?
Usually not. Published emission factors for fuel, electricity and travel are already expressed in kg CO₂e, with the small amounts of methane and nitrous oxide from combustion built in. You multiply your activity by the factor and the conversion is done for you. Our guide to emission factors explains how those numbers are put together.
The one case where a small business applies GWP directly is leaked gas, most often refrigerant. Take the kilograms topped up from the service record, look up the GWP of that specific gas, and multiply. The result goes in Scope 1. See Scope 1, 2 and 3 explained for where each source belongs, and how to calculate a business carbon footprint for the full method.
What CO₂e does not tell you
CO₂e is a useful simplification, and it has limits. It compresses gases with very different lifetimes into a single number, so it hides when the warming happens. It also says nothing about other environmental effects, such as air pollution, water use or damage to habitats. Treat it as the standard measure of climate impact, not as a complete measure of environmental impact.
Frequently asked questions
Is CO₂e the same as a carbon footprint?
CO₂e is the unit. A carbon footprint is the total, measured in that unit, for a company, product, person or activity.
How is CO₂e written?
CO₂e, CO2e, CO₂eq and CO₂-eq all mean the same thing.
Why does methane have two values?
The Sixth Assessment Report gives a slightly higher figure for methane from fossil sources, because the carbon dioxide it eventually breaks down into is new to the atmosphere. For methane from biological sources that carbon was recently absorbed by plants.
How much is one tonne of CO₂e?
As a rough guide, burning about 430 litres of petrol releases one tonne of carbon dioxide.
Sources
- IPCC Sixth Assessment Report, Working Group I
- GHG Protocol Corporate Accounting and Reporting Standard
About the author
Georgi Todorov is an eco activist and entrepreneur. He runs Ecoki, where he builds free tools and guides that help small businesses measure and cut their carbon footprint.