A range of carbon footprint calculators have emerged over the years, aiming at promoting pro-environmental behaviour through providing information about what impact people have on the environment. Up until recently, most of these calculators have been focusing on providing feedback on an individual level. This paper presents an exploratory study of a new kind of carbon footprint calculator, which offers a social and collective dimension not found in many other existing calculators. This is done through the introduction of a group feature allowing people to engage with and compare themselves to each other. The calculator also makes use of real-time financial data in combination with user generated data in order to provide reliable and continuous estimates of a person's carbon footprint. Through an explorative study, in which we conducted two in-depth interviews with four participants, we have investigated the reactions to using the carbon calculator for the first time as well as after two to three weeks of unsupervised use. Our study indicates that the use of transaction data does not automatically lead to a higher trust in the calculated carbon footprint due to the numerous insecurities that are revealed. Registry data on the other hand seems to be appreciated because it eases the input that people have to provide anyway. While groups seem to be a promising feature, there is a need to investigate what information about people's carbon footprints should be shared as well as how the groups and the interaction with the carbon calculator can be kept lively and interesting over time.