To calculate your footprint for a given transaction, we assign an emissions factor to all the industries you might buy from and activities you could pay for (fashion, travel, energy, grocery, insurance, etc). Then, each transaction is multiplied by the emissions factor to calculate its impact. For example, £1 spent at a UK fashion retailer creates on average 0.9kg of CO2e.
Most of these calculations don’t take into account the difference between the same products bought from different companies. But for transactions with some types of business, like energy suppliers or second-hand fashion retailers, the estimate can be more precise to the company.