In the Belgian building acoustical standards, two different descriptors are currently used in the requirements for the impact sound insulation, depending on the building type. On the one hand, the weighted standardised impact sound pressure level L'nT,w is used, on the other hand the descriptor L'I = L'nT,w + CI. To simplify the standards, one is considering to express all requirements using the descriptor L'I. For this reason, the translation between the two single number quantities was investigated for different types of floor constructions. Empirical relationships were derived from a statistical analysis performed on a large database of laboratory measurements extended with simulations for heavyweight reference floors. A strong correlation is found between the descriptors LnT,w and LI when the floor constructions are subdivided in the following three categories: i) heavyweight reference floors, ii) lightweight reference floors, and iii) heavyweight or lightweight floors with floor coverings and/or suspended ceilings. For each category, the correlation coefficient between the two descriptors is larger than 0.97. Furthermore, the LI-value of heavyweight reference floors can be accurately estimated from the floor mass per unit area. A clear correlation is also revealed between the single number quantities describing the reduction in impact sound level, e.g. by floor coverings or suspended ceilings.