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The rank of clutter covariance matrix is an important indicator of both the severity of the clutter scenario and the number of degrees-of-freedom required to produce effective clutter suppression. Thus, the estimation of clutter rank is an essential issue for space-time adaptive processing. The rank of ground clutter covariance matrix can be accurately predicted by the well-known Brennan's rule. Unfortunately, the similar rule for ocean clutter rank estimation has not been developed. In this paper, a formula to estimate the rank of ocean clutter covariance matrix is derived, with which the eigenspectrum structure of ocean clutter can be correctly predicted. The validity of this formula is verified by simulation results.