Carbon calculators have been put forth as a tool to motivate sustainable behaviour change in people. However, the approach of “just” presenting numbers to communicate climate footprints has not been found to be an effective strategy. In this paper, we investigate the use of an application that combines carbon footprint calculations with gamelike features in order to address the gap between awareness and behaviour. Our results are based on an interview study and show that while respondents appreciate the idea, there are several problem areas which have implications for the design of carbon calculators, including issues with targeting the “right users”, the use of gamification and the absence of a social context. Furthermore, the results point towards general barriers and opportunities for design when the aim is to design for sustainable behaviour change. This includes a need to be adaptive to the transitioning process towards a low carbon lifestyle.