CO₂e is the abbreviation for "carbon dioxide equivalent."

Carbon dioxide equivalent is the standard measurement of greenhouse gas emissions in terms of the most common greenhouse gas, which is carbon dioxide (CO₂).

Different greenhouse gases have different heat-trappping abilities and different atmospheric lifetimes. So they contribute to global warming in different ways and on different scales.

For example, one tonne of carbon dioxide causes a different amount of warming than one tonne of methane over the same time period.

It would be cumbersome if we had to list separate measurements for every single greenhouse gas any time we discuss the emissions that contribute to global warming.

So we use carbon dioxide as the common denominator, a benchmark to which we can compare the effects of other greenhouse gases.

For example, to determine the carbon dioxide equivalent of methane, we take the amount of warming that a unit mass of methane would generate over 100 years, then figure out the unit mass of carbon dioxide that would generate the same amount of warming over 100 years.

Fortunately, the Intergovernmental Panel on Climate Change (IPCC) has already done this research, so we know that releasing 1 kg of methane is equivalent to releasing 25 kg of carbon dioxide.

This video from the University of Queensland uses currency conversion as a helpful analogy. We convert the global warming potential of methane to a carbon dioxide equivalent the same way we might convert the value of euros to dollars.

Per Climatiq, an emission factor is a value that estimates the quantity of a gas with a global warming potential (GWP) released into the atmosphere, per some unit related to a particular activity.

For example, one emission factor we use is:
passenger_vehicle-vehicle_type_car-fuel_source_na-engine_size_na-vehicle_age_na-vehicle_weight_na

This is the emission intensity of a passenger car (any vehicle with a wheelbase less than 121 inches, which includes passenger cars/minivans/SUVs/small pickup trucks).

For every mile driven, 0.343372 kg of CO₂e is emitted.

You can explore other emission factors here.

We use the Climatiq API, which allows us to access a database of up-to-date and scientifically vetted emission factors.

Climatiq provides emission factors such as the kg of CO₂e generated when one person travels one mile by car in the United States.

When you enter a number of miles, we multiply that value by the CO₂e kg value provided by Climatiq.

When you enter an origin and destination, we use the Google Maps Places API and the Google Maps Distance Matrix API to calculate the distance between them.

Google sends back the distance between those two places in meters.

We convert the meters to miles, then multiply that value by the CO₂e kg provided by Climatiq.

For more detail, you can view the code here on GitHub.

Emission factors vary by region.

A train ride in the UK, for example, might emit a different amount of CO₂e than a train ride in the US.

Every project has to start somewhere, and since the US is a major producer of carbon emissions, it made sense to start there.

We may add estimates for other emission factors in the future.