A method of calculating the heart rate variability of the human heart to be used in an ECG monitor comprises the following procedural steps: scanning of the ECG signal received from the ECG monitor during a scanning interval, determining from the scanned ECG signals a number of discrete measuring values representative of the heart rate variability, and evaluating these measuring values on the basis of the Fourier transformation, wherein the frequency spectre of the measuring values is calculated from the Fourier coefficients of the Fourier transformation, which are for their turn calculated from a combination of said measuring values with the sinus- and cosinus-shaped Fourier vectors of the Fourier transformation wherein the Fourier vectors are involved in the calculation in the form of a number of discrete real numerical vector values, and replacing of the numerical vector values for calculating the frequency spectrumof the scanned measuring values by rough, integral approximate vector values of a limited number which roughly form the shape of the Fourier vectors.