A pre-condition for reducing global warming is to minimise the emission of greenhouse gasses (GHGs). A common approach to informing people about the link between behaviour and climate change rests on developing GHG calculators that quantify the ‘carbon footprint’ of a product, a sector or an actor. There is, however, an abundance of GHG calculators that rely on very different premises and give very different estimates of carbon footprints. In this chapter, we compare and analyse the main principles of calculating carbon footprints, and discuss how calculators can inform (or misinform) people who wish to adopt greener behaviour.