A calibration of a focus error signal gain in a focus servo system of an optical disk drive is presented. The focus error signal gain can be calibrated by oscillating an optical pick-up unit in a focus direction through a focus point and determining a focus sum threshold. The optical pick-up unit can then be oscillated around a focus control effort that results in a sum signal equal to the focus sum threshold and the peak-to-peak value of a focus error signal can be measured. The focus error signal gain can be determined from the peak-to-peak value of the focus error signal.