Method for detecting bearing defect severity based on bearing rotation speed and at least one data sample of sensor data obtained to measure vibrations of a bearing ring is provided. The method includes converting the data sample from time domain to frequency domain to obtain a signal frequency spectrum; determining a defect center frequency of the bearing using the rotation speed; and identifying a predetermined number of frequency peaks of the signal frequency spectrum. A total vibration energy in an overall frequency band including the predetermined number of frequency peaks is first determined; next, for each of the frequency peaks, a peak energy as a spectral energy of signal components giving rise to the frequency peaks is determined; calculating a bearing defect spectral energy using the peak energies; and finally a ratio of the bearing defect spectral energy and a total vibration energy to assess a defect severity is obtained.