What is CO2e kg?

Updated: Mar 4

With the rise in companies and countries all over the world calculating their greenhouse gas emissions (GHGs), measuring Carbon Dioxide Equivalent (CO2e) in kg is becoming increasingly commonplace.

But what is CO2e and how can gases be measured in kg?

Greenhouse Gases (GHGs) have created large-scale imbalances in our environment, as they are being generated faster than they can be sequestered. The gas most often quoted as being most impactful on Global Warming and Climate Change is Carbon Dioxide (CO2) but many other gases also play their own role, including,

  • Methane(CH4)

  • Nitrous Oxide (N2O)

  • Hydrofluorocarbons(HFCs)

  • Perfluorocarbons(PFPs)

  • Sulphur Hexaflouride (SF6)

All GHGs allow sunlight to pass through the atmosphere, but then trap the excess heat from that sunlight, stopping it from being able to leave, and turning the atmosphere into a kind of greenhouse with the Earth sitting in the middle, hence the name. However, all GHGs ‘trap heat at different rates and have different lifetimes in the atmosphere, making their contributions to the Greenhouse Effect extremely diverse.’ [1] This leaves scientists with a problem then, how can they compare the impact of all of the different gases, when GHGs all working to different degrees, for a different number of years in our atmosphere?

Global Warming Potential (GWP)

To adequately compare all the other GHG emissions sources, CO2 has been chosen to become the main comparison, due to its increased prominence in our atmosphere. All other gases are then expressed as CO2e, or ‘Carbon Dioxide Equivalency’ allowing for a common unit and ease of understanding between inter and intra industry comparison and understanding.[2]

Each gas has a specific global warming potential (GWP), which allows comparisons of the amount of energy the emissions of 1 tonne of a gas will absorb over a given time period, usually a 100-year averaging time, compared with the emissions of 1 tonne of CO2.[3]

GHG GWP potential is calculated by scientists based on the current quantity in the atmosphere, and the lifetime of the gas when in the atmosphere.

Each gas has a unique GWP based on its unique concentrations, but by referring to them using a common unit, we can more accurately compare the damage they are capable of, and therefore assess the seriousness of each gas throughout its lifetime, making it easier for people to act on this information. For example, CO2 has a GWP of 1, while CH4 has a GWP of 25, meaning that every tonne of methane is equivalent to 25 tonnes of CO2, which goes some way to highlight the danger of the gas, and therefore the increased need to remove it.

This gives scientists and governments the ability to take a baseline measurement, allowing for future comparisons once change has been implemented, and gives them an idea of what to prioritise when trying to decrease emissions.

The chart [4] above demonstrates the approximate GWP for each gas and gives a handy indication of which gases are naturally occurring, including CO2, CH4 and N2O, as they have measurements available from before the rise of human industry. It also evidences which harmful gases are anthropogenic, man-made, for example, HFC-23, CF4, and SF6 as they have no pre-industrial concentrations.

How can we calculate the amount of carbon dioxide in kg?

Being able to calculate the amount of GHGs is a complicated affair. It involves an understanding of atomic weights, that is the mass of an atom. Once we know these weights, scientists can calculate what the mass of the GHG is, based on the original mass of atoms. To calculate CO2 emissions, for example, we can start with the original mass of carbon atoms.

We can see that a carbon atom has a weight of 12au.

Once combustion occurs, 1 carbon atom ‘C’ will combine with 2 oxygen atoms ‘O,’ creating this equation,

C + O2 becomes 12au + (16aux2) = 44au

‘The addition of two oxygen atoms to each carbon atom forms CO2, which has an atomic weight of 44—roughly 3.6667 times the atomic weight of the carbon, which is 12.’[5]

Therefore we can understand that burning 1kg of carbon results in roughly 3.7 kg of CO2 production.

We can then use this understanding to calculate the mass of CO2 in petrol.

For example, if 1L of petrol weighs 750g, and contains 87% carbon and therefore the weight of carbon is 652g per L. To complete combustion, and turn C into CO2, 1,740g of oxygen is needed, resulting in this equation,

652g ‘C’ + 1,740g ‘O’ = 2,392g, or2.39kg of CO2/L.[6]

So, using the original masses of carbon and oxygen in grams, we can perform simple arithmetic and consider the combined weight of both elements, giving us the weight of CO2, noting, for example, that 1L of petrol will generate 2.39kg of CO2.

This is applicable for all of the GHGs, as we can use their known atomic weights and concentrations within substances to calculate their mass in gas form in kg.[7]

As we are increasingly finding in our worldwide journey to increased sustainability and carbon neutrality, considering the key players, GHG emissions, as equivalences and having the ability to compare them per kg, makes the effort to understand, and the efforts to remove them much easier.

It also allows the public, companies, and countries across the world to have a baseline of understanding, which can be built on to compare complicated concepts, and gives us more of a chance of reducing harmful emissions and halting the progress of Climate Change.

All thanks to carbon dioxide equivalency and standardised units!

So, let us know in the comments, what surprised you most?

Was it the ease of measuring gas with units generally associated with solid mass?

Or the reasoning behind expressing GHGs as CO2e?


[1] University of Arkansas Sustainability Blog, ‘Definition: GWP and CO2e,’published by Linden Cheek, 25.05.2016,

[2]Green Element.co.uk ‘Energy and Carbon Footprint Benchmarking,’ <https://www.greenelement.co.uk/benchmarking-carbon-footprinting/>09.02.2022

[3] Air pollution biogeochemistry, published by Daniel A. Vallero, in Air Pollution Calculations, 2019, available online at Science Direct.com <https://www.sciencedirect.com/topics/earth-and-planetary-sciences/global-warming-potential>

[4] University of Arkansas Sustainability Blog, ‘Definition: GWP and CO2e,’published by Linden Cheek, 25.05.2016,

[5] U S Energy Information Administration.gov, ‘FAQs,’ last reviewed 04.02.2021, <https://www.eia.gov/tools/faqs/faq.php?id=82&t=11>

[6] Ecoscore.be, ‘How to calculate the CO2 emission from the fuel consumption?’ <https://ecoscore.be/en/info/ecoscore/co2>09.02.2022

[7] Image from Pexels.com, courtesy of Szabolcs Toth.

Cover image from Pexels.com, courtesy of Pixabay.

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