Determination of the detector effi ciency using volumetric cylindrical sources is very important in various scientifi c and industrial fi elds, especially in the fi eld of quantitative analysis. To calculate the absolute activity of any sample, the full-energy peak effi ciency (FEPE) of the detector is needed. By applying the effi ciency transfer method, the FEPE of the detector would be determined easily without using the standard sources. This approach depends on two main factors. The fi rst one, is the reference effi ciency of the reference source, which is determined experimentally, and the second one, is the calculation of the effective solid angle ratio between the sample and the reference source geometries. This work introduces an empirical formula for calculating the second factor for using two different sizes of NaI(Tl) detectors. The validity of this empirical formula was successfully demonstrated by comparing the calculating values with the experimental values.