Carbon calculators can potentially help people understand and reduce their climate impact. This paper describes the features of Svalna, a mobile application that estimates users’ greenhouse gas emissions by means of a hybrid approach that relies on financial transaction data from the users’ bank paired with environmentally extended input output analysis; data from official registers of governmental agencies, and data entered by the users themselves. Applying financial transaction data in carbon calculators is increasingly popular, and I discuss the benefits and drawbacks of using such data in order to estimate greenhouse gas emissions. The paper also describes how design features have been developed based on insights from behavioral science and previous research on carbon calculators. The mobile application is available for use in Sweden and had approximately 15,000 users in December 2019.