A simple accurate model has been devised for ocean surface duct propagation. The model assumes the following form for the propagation loss: P.L. = 60 + 10 logrT + 10 logr − “gain,” where the transition range from spherical to cylindrical spreading, rT, is computed from the limiting ray for the duct, and the “gain” term is computed using the normal modes of the duct, as a ratio of the coherent mode sum to the incoherent sum. Complex reflection coefficients at the “turning points” for each mode determine the leakage of energy out of the duct due to diffraction and scattering; the (real/imaginary) parts of these reflection coefficients determine the (imaginary/real) parts of the normal mode eigenvalues. Absorption losses are added in an ad hoc fashion for each mode. This model is shown to be equivalent in an approximate way to the complete Green's function solution to the wave equation which includes both the discrete and continuous portions of the eigenvalue spectrum.