How are CO2 emissions calculated?

The principle of GHGs emissions quantification for an organization is simple, for each emission source, the following calculation is performed.

Activity Data

Activity data

The activity data from a company corresponds to the quantity of a specific product or service (e.g., amount of kWh of electricity used, numbers of cars, tons of product etc.).

Custom Emission Factors

Emission factor

The emissions factor is specific to each activity data and therefore allows to convert the activity data into the quantity of CO2 equivalent emitted (for example: Burning 1 litre of petrol=2.3 kg CO2e).

These emission factors are available in various carbon databases such as, among others, the ADEME database or the IPCC.


Estimating emissions inherently involves some uncertainty. This is due to the challenges in gathering precise data and the potential variability in emission factors, both of which contribute to the overall calculation.

See how uncertainty is calculated.

Activity data

The uncertainties related to the activity data are allocated by the user of the tool or by the tool according to the quality of the provided data.

To guide the choice, the uncertainty on the activity data can be standardized as follows.

Type of data Uncertainty
Direct internal data (invoices or meters) 5%
Internal data processed (e.g. conversion) 10%
Internal data obtained on the basis of hypothesis 20%
Internal data not available, data obtained on the basis of a benchmark. 50%

Emission factor

Uncertainties related to emissions factors (UFE) are directly defined by the Bilan Carbone® methodology or the GHG protocol methodology.

The total uncertainty of an emission source is defined by:
UPost = √(UFE2)+(Udata2)

UFE: the uncertainty of the emission factor
UData: the uncertainty of the respective data at each position
UPost: the overall uncertainty of the position

UPost is then multiplied by the emissions (in CO2e) of the source concerned.

The product of the uncertainties of each source and the emissions of each corresponding source gives the total uncertainty (in CO2e).

Note that the confidence interval used in GHG inventories is 95%; this confidence interval is the interval centered on the value retained for a data in which it is 95% likely that the true value of this data is included.