Efficiency calibration by standard materials of known activity is usual protocol for γ-ray spectrometry. In general, 40K activity is measured from proper efficiency calibration covering broad energy range within which the 1460.83 keV photopeak falls. However, less frequently, the activity of 40K is reported by comparing the area under 1460.83 keV photopeak of natural sample to that of standard. The present study shows that determining the activity of 40K by efficiency calibration is highly dependent on the nature of fitting of calibration curve. The present paper also shows that comparator methods are more reliable for reporting 40K activity than efficiency calibration. © 2018, Akadémiai Kiadó, Budapest, Hungary.