The recommended levels of ionizing radiation from construction materials in effective dose is set to a maximum of 1 mSv/year, EC (1996, 1999, 2013), ICRP (2007), IAEA (2011). By using a theoretical model proposed by the European Union (1999), this is equivalent to I-index 1. By using of concrete slabs with dimensions of 1.5 m × 1.5 m × 0.15 m, an empirical approach is suggested for the calculation of the I-index of naturally occurring ionizing radiation from construction materials. Measurements of 40K, 226Ra, 232Th and the total gamma radiation were conducted and the I-index values were calculated for each concrete mix. A good linear relationship could be established between measurements performed by the Swedish Cement and Research Institute (CBI) and the laboratory results acquired from the Radiation and Nuclear Safety Authority of Finland (STUK) and Centre de Recherches Pétrographique et Géochimiques/ Le Centre National de la Recherche Scientifique (CRPG/CNRS). The results indicate that 60 % of the investigated construction materials are in agreement with the stipulated levels set out by the EC (1999, 2013). The cause for the higher levels of ionizing radiation is often elevated concentrations of all the radioactive elements measured. Regarding the concrete samples yielding values of I-index > 1, 232Th makes the largest contribution.