An approximate dependence between cavitation numbers in an unbounded flow and in an experimental section of a water tunnel, at which the equality of the maximum transverse dimensions of the cavities formed behind identical cavitators is ensured, is obtained in the framework of a model of a viscous, weightless, incompressible liquid. On the basis of an analysis of the well-known numerical calculations of developed cavitation flows for cavitators of different shape in the two-dimensional and axisymmetric cases, and those carried out by the authors, an estimate is made showing that when the found relation between these cavitation numbers is satisfied the relative lengths, the relative maximum transverse dimensions, and the elongations of the cavities are also equal in unbounded and bounded flows. These values are equal in the considered cases, correct to 6%, for all the cavitation numbers in the tunnel which differ from the limiting values by not less than 5%. This conclusion is verified by experiments of the authors and other investigators.