The energy consumption of information and communication technology (ICT) is still increasing. Even though several solutions regarding the hardware side of Green IT exist, the software contribution to Green IT is not well investigated. The carbon footprint is one way to rate the environmental impacts of ICT. In order to get an impression of the induced CO2 emissions of software, we will present a calculation method for the carbon footprint of a software product over its life cycle. We also offer an approach on how to integrate some aspects of carbon footprint calculation into software development processes and discuss impacts and tools regarding this calculation method. We thus show the relevance of energy measurements and the attention to impacts on the carbon footprint by software within Green Software Engineering.