CO₂e, also sometimes written as CO₂-equivalent, CO₂eq or other variations, stands for carbon dioxide equivalent.

Different greenhouse gasses have different global warming impacts for the same quantity of gas. For example, 1kg of methane (CH₄) has a global warming impact that is 25 times that of 1kg of carbon dioxide (CO₂).

For any quantity and type of greenhouse gas, there is a CO₂e number that indicates the amount of CO₂ that would result in the equivalent global warming impact. Therefore, 1kg of methane is 25kg of CO₂e.

CO₂e then, is a standard unit that can be used to compare emissions side by side, even when those emissions are comprised of different greenhouse gasses.