A new concept was proposed to calibrate gamma spectrometers for measuring the activity of volumetric samples. The concept used a formula that was developed to give a correction factor to the detector efficiency when the detector was calibrated with a point source. The concept gave efficiency curves for an HPGe detector. The results obtained by the concept yielded deviations between 0.5 to 5% from the experimental data. The concept needs only the availability of the MCNP code or similar gamma transport codes.