The objective of this paper is to develop a device to measure the k∞ of a spent fuel assembly used in light water reactors. A subcritical assembly having a cross configuration is designed to allow measurement of the k∞ of a spent fuel assembly by comparing the change in its multiplication with that of a fuel assembly of known k∞. Calculations have been performed using nucleonic codes to develop polynomial equations that relate the k∞ of the spent fuel assembly to measured data. The measurements involve taking count rates with the spent fuel assembly in the center position of the subcritical assembly, and the measured data are the count rate ratio of the spent fuel assembly over the count rate taken with a fuel assembly of known k∞. The polynomial equations are easy to program on a microcomputer, which, together with the subcritical assembly, form the k∞ meter.