We need three bits of data to calculate your carbon footprint: transactions, business names and emissions factors (EFs).
Once we have sorted your spend into categories based on business names, the Carbon Tracker compares those categories against a comprehensive list of business types and their EFs.
An EF is the amount of CO2 produced by £1 of spend, for a particular type of business. For example, a trip to a supermarket will on average produce 1.01 KG of CO2e per £ spent (less if you're a vegetarian). Extensive research has been conducted to calculate EFs that are as accurate as possible.
Despite considerable effort, the EFs still only represent the average emissions per transaction. We constantly work to improve their accuracy, for example by allowing companies to provide their own verified data.