This study examines the relationship between the strength of concrete and its environmental impact, particularly focusing on carbon dioxide emissions, with consideration of the use of reinforced concrete in building constructions. By constructing a six-story building with varying concrete strength classes, their carbon dioxide emissions and dimensions in building design are compared. The question is whether the use of higher strength concrete, which requires more cement and thus generates more CO2 during production, can be compensated by savings in transport and steel due to the reduced volume of concrete and steel. The aim is to provide insights that can guide decision-makers in the construction industry when choosing concrete strength for different projects and to investigate how the strength of concrete affects carbon dioxide emissions, with the goal of contributing to more sustainable building practices. The result shows that CO2 emissions during production can be compensated by savings in transport and steel, where design 1 with lower concrete compressive strength showed 13 % more CO2 emissions compared to design 2, which was designed with higher concrete compressive strength class.