Cement manufacture is contributing to CO2 emissions as calculated by the Intergovernmental Panel for Climatic Change (IPCC) of 520 kg of CO2 per tonne of clinker. When the concrete is produced, hydrated cement phases in concrete absorb CO2 through the carbonation of cement paste. The uptake of CO2 with respect to the CaO content in the cement is named as degree of carbonation, DoC. The calculation of that DoC or “recarbonation” of concrete has been assumed as a fixed percentage in most of the studies that were undertaken in the past for calculating the carbon footprint. However, the DoC may vary with the cement type, the concrete mix proportion and the particular exposure class. These values of DoC were calibrated in a Spanish study with 15 different cements, 4 concrete mix proportions and 3 exposure classes. Present paper comments on the concept of DoC and summarizes the experimental work made in Spain. The statistical analysis of the results indicated an average value of the degree of carbonation (DoC) for all cement types, except those containing high proportions of GGBS, of around DoC = 50%. This value is different from some recommended in the Annex BB of standard EN 16757 for the European Environmental Declarations.