If the carbon footprint is the metric of global warming, CO2e is its unit of measurement.

CO2e or CO2 equivalent is the unit of measurement for the impact of different greenhouse gases (GHGs) on global warming, expressed in terms of the amount of CO2 calculated on the basis of the Global Warming Potential index.

Thanks to this unit of measurement, we can calculate with a single value the environmental impact that an activity, event or product would cause by emitting many different greenhouse gases, considering them in the equivalent amount of carbon dioxide.

The United Nations Intergovernmental Panel on Climate Change (IPCC) has developed the Global Warming Potential (GWP): an index that measures the warming potential absorbed by a given amount of a greenhouse gas in comparison with that absorbed by the same mass of CO2 over a period of 100 years.

The main greenhouse gas is carbon dioxide (CO2), emitted every time fossil fuels are burned. It is followed by methane (CH4), produced mainly by agriculture and landfills, and nitrous oxide (N2O), released mainly by industrial and agricultural processes. Refrigerant gases (HFCs) and sulphur hexafluoride (SF6) are present in smaller quantities in the atmosphere, but have a climate impact enormously greater than CO2.

The Formula to calculate the CO2 equivalent

You multiply the mass of the gas by its Global Warming Potential (GWP):

CO2e (t) = mass of gas (t) × GWP

The Global Warming Potential is a fixed value that expresses any greenhouse gas in terms of CO2.

A practical example: an agricultural company emits 10 tonnes of methane (CH4) and 0.5 tonnes of nitrous oxide (N2O) in a year.

The result — 412.5 t CO2e — is a single value that can be compared with other organisations and reported according to GHG Protocol and ISO 14067.

CO2e conversion tables are available on the UNFCCC website and on GHG Protocol, based on the IPCC Fifth Assessment Report.

A single production process can cause the emission of a combination of different greenhouse gases in varying quantities. In these cases, calculating the carbon footprint can become quite complicated without a common unit of measurement.

To simplify the study of greenhouse gas emissions and make the emissions from different types of gases comparable, the CO2 equivalent (CO2e) is used as the standard unit of measurement.

The difference between the two terms is therefore straightforward:

• CO2 refers exclusively to carbon dioxide emissions
• CO2 equivalent (CO2e) encompasses the impact of all greenhouse gases, converted to their CO2-equivalent value through the GWP

CO2 equivalent is not just a technical unit of measurement: it is the foundation of all corporate emissions measurement and reporting processes.

Without CO2e it would not be possible to:

• compile a GHG Inventory according to GHG Protocol (Scope 1, 2 and 3)
• calculate the corporate or product carbon footprint according to ISO 14064 and ISO 14067
• start a journey towards carbon neutrality certified under ISO 14068
• purchase certified carbon credits to offset residual emissions
• produce a sustainability report according to GRI 305 or CSRD/ESRS E1

In practice, every time a company measures its climate impact — whether for a sustainability report, an environmental certification or a Net Zero target — it expresses the result in tonnes of CO2e.