The steps of calculating the carbon footprint of a supply chain is straightforward:
1- You need to map out your supply chain. This also defines the boundary of your calculation.
2- Collect activity data at each level of your supply chain. For example, an oil company may start at the exploration and drilling stage, and collect data on how much energy it is paying for those activities: mileage traveled, fuels burned, electricity consumed, etc..
3- Collect data on emission factors. This is probably the easiest step, because such data as CO2/KWh and CO2/mile are public available. This is also a key step because it represents the technology mix of the present and of the future.
4- Multiple activities with the emission factors will give you the amount of GHGs emitted at each level of your supply chain. Because GHGs contain multiple types of gases, a common way is to use a ratio to convert different types of gases into CO2e – Carbon Dioxide Equivalent.
That is it!
Here is a simple video tutorial: