An onboard system and method for determining the instantaneous weight and balance of an aircraft simply, reliably, accurately, and requiring a minimum amount of calibration includes a memory for storing previously determined breakout friction data of the aircraft's landing gear shock struts, sensors for sensing the pressures in the struts, the vertical loads exerted by the landing gear on the aircraft, and the attitude of the aircraft relative to the horizontal during loading or unloading thereof, and a computer for computing the vertical load in each of the landing gears from the stored calibration breakout friction data and the shock strut pressures, landing gear vertical loads and aircraft attitude sensed during the loading or unloading. The computer then computes the gross weight of the aircraft and the location of its center of gravity (CG) using the computed vertical loads in the landing gears.