The method we use for our product footprints is based on how much we spend each month on new goods. This expenditure is divided up into six different product groups from which we estimate an emissions factor that captures the emissions arising from the materials, manufacturing and distribution associated with each product group .
Read more at http://shrinkthatfootprint.com/calculate-your-carbon-footprint#FTvp05oM57c53X8l.99