Problems concerning carbon dioxide emissions and other climate change-related issues are on the global political agenda and constantly debated in media. Such issues are important for individuals to enable active participation in society. This study has a particular interest in the use of carbon footprint calculators (tools for calculating carbon dioxide emissions of human activities) in the context of learning about environmental issues and climate change. More specifically, it contributes with insights into how such tools foster different modes of reasoning about the environment. The empirical data consist of video recordings of 15 Swedish upper secondary students' classroom discussions. The study derived from one specific half-day-lesson with activities related to the use of a carbon footprint calculator. In the first part of the lesson, the students worked individually with the tool for calculating their carbon footprint, and in the second part of the lesson, the students discussed their carbon footprints in groups. The focus of the analysis is on the group discussion and on what modes of reasoning and arguing about the environment that are made possible through the students' use of the calculator. The study investigates the students' accounts in relation to how they discuss and compare their carbon footprints. That is, how the students in their discussions explain and justify actions in their everyday lifestyle. The findings indicate that the carbon footprint calculator supports different modes of reasoning and arguing about the environmental impact of actions in students' everyday lifestyle. The carbon footprint calculator offers students a new arena for developing an understanding of climate change and its relationships to human activities. The results shed light on the ways in which students are able to quantify, analyse and compare carbon dioxide emissions both on an individual level but also at a systemic level (across countries) after having used the carbon footprint calculator. The tool thus mediates features of the environment that students otherwise could not perceive; it makes the invisible visible.