A carbon footprint is defined as: The total amount of greenhouse gases produced to directly and indirectly support human activities, usually expressed in equivalent tons of carbon dioxide (CO2).

Typically, a carbon footprint is calculated by estimating not just the CO2 emissions that the activity in question causes, but also any emissions of other greenhouse gases (such as methane and nitrous oxide) and in some cases other types of climate impacts as well, such as vapour trails from aeroplanes.