How is a product's carbon footprint calculated?

ISO14040 describe the principles and framework for life cycle assessment and can also be used to calculate specifically the carbon footprint of products. There currently are no ISO standards dealing specifically with the carbon footprint of products.

Greenhouse Gas Protocol (GHG Protocol)

The Greenhouse Gas Protocol Initiative, commonly called "GHG Protocol" is among the most widely used international accounting tools for assessing and managing GHG emissions at the corporate level, the project level and now at the product level. Initiated in 1998 by the World Resource Institute (WRI) and the World Business Council for Sustainable Development (WBCSD), the GHG Protocol was developed in partnership with businesses, NGOs and governments. It provides a wide set of resources, tools and data for carbon footprint calculations. Its main characteristic is that it is adapted to just about every accounting system.

Methodology proposed by the GHG Protocol allows for the full and transparent communication of an organisation’s GHG emissions. It calls for the entry of all data pertaining to direct energy consumption from sources owned or operated by the company as well as direct energy purchases (electricity, heating, air conditioning). A broader scope option is also proposed in order to include the indirect emissions that occur upstream and downstream of the company or product value chain.