Abstract: A stability indicator for a marine vessel. The indicator continuously determines a GM value for the vessel equal to the square of the vessel's beam length times the square of the vessel's rolling constant divided by the square of the vessel's rolling period. The rolling period is determined by means of a gravity sensor whose output is an electrical signal corresponding to the instantaneous magnitude and direction of the roll angle. Electronic level adjustment and gain selection means are also provided, to compensate for the fact that the stability indicator may often be installed with the gravity sensor off-horizontal. The signal from the gravity sensor is input into a data processor that computes the rolling period by first analyzing the roll angle signal to establish a zero level and threshold levels above and below the zero level.