In standard engineering practice, designers often aim to minimize the amount and volume of concrete in reinforced concrete (RC) frames by reducing the size of member cross-sections. This is done either for architectural reasons or because it is assumed to drive to more economic and environmentally friendly productions. The present study, for first time, compares the environmental footprint of RC frames designed for minimum concrete volume against designs for minimum embodied carbon. To serve this goal, six realistic 3D RC building frames are optimally designed for parametric values of the carbon factors of concrete and reinforcing steel materials. In this comparison, it is noticed that the carbon factors ratio R of reinforcing steel to concrete plays a key role. More particularly, it is found that for R ≤ 10 the designs for minimum concrete and carbon practically coincide. This is a useful observation since it signals a clear direction to designers to decrease concrete sections to achieve minimum environmental impact. Nevertheless, as R increases from 10, the two designs gradually deviate since the carbon footprint of rebars becomes more important. For high R values, the RC frames with the least amount of concrete produce, on average, up to 40 % more embodied carbon than the most environmentally clean designs.