CO2 Breathing Emission Calculator
About a kilogram a day, and it contributes nothing — the carbon came from food, not from the ground.
The formula
g CO2 per day = litres per minute * CO2 fraction * 1440 * 1.977
About a kilogram a day
An adult breathes roughly eight litres of air a minute averaged over a day and returns it with about 4 % carbon dioxide, which works out at close to a kilogram. Exercise raises it several-fold for as long as it lasts, and sleep lowers it.
And it does not count
This is the important part. The carbon you exhale came out of the atmosphere within the last year or two, through plants and then through your food. Breathing returns it to where it came from, and the cycle is closed. It adds nothing to atmospheric CO2 and never has.
Fossil fuels are different in exactly one respect that happens to be the only one that matters: their carbon has been out of circulation for tens of millions of years. Burning them introduces carbon the current cycle has never held. That distinction — recent carbon against ancient carbon — is the whole of why breathing is neutral and driving is not, and the argument that human respiration is a climate problem rests on ignoring it.
How much to trust a footprint
Life-cycle figures for the same product routinely differ by a factor of two between studies, because deciding what to count — the farm, the packaging, the shop, the drive home, the disposal — is a methodological choice rather than a measurement. The numbers here are central estimates from published meta-analyses. They are reliable for comparing one option against another, which is what people actually use them for, and not reliable to the second significant figure.
The other thing worth keeping in view is scale. Individual choices matter and they are not where the emissions are: the difference between the best and worst quartile of beef producers is larger than most people's entire dietary footprint, and that is a question about agriculture policy rather than about shopping.