Coherence bandwidth is an important characteristic of multipath fading channels, serving as a useful tool for wireless systems design. Regardless of its importance, the determination of the coherence bandwidth, so far, has relied on an empirical formula. In this correspondence, we derive an exact coherence-bandwidth formula for Rayleigh fading channels. The use of the new formula is illustrated by numerical results. Index Terms-Coherence bandwidth, multipath channels, power delay profile (PDP), Rayleigh fading channels.