Time-of-flight range imaging systems illuminate a scene with an amplitude-modulated light source, the light is reflected from objects in the scene, and measurement of the phase of the modulation envelope is performed to determine the object’s distance. As the image sensor is capable of performing this task for every pixel simultaneously, acquisition of the entire scene can be performed at rapid (video) update rates, making the technology ideal for real-time applications. In this paper we present an efficient real-time FPGA algorithm for determining phase, and hence distance, from the raw image sensor output. The algorithm has been implemented on a range imaging system based on the PMD19k-2 image sensor, with range processing performed in real time by a Stratix III FPGA. The scarcest resource in this implementation is RAM, and an analysis is presented to maximise the efficiency of this resource whilst maintaining acceptable processing accuracy. The algorithm can be extended for processing multiple simultaneous modulation frequencies. An efficient method for combining these results to determine unambiguous range, based on the Chinese remainder theorem, is also presented.