The complete derivation of two-dimensional (2-D) coupled wave equations is given for total overlap gratings in the Bragg regime. The analytical solution in closed mathematical form is also given. General diffraction properties of total overlap gratings, can be solved with the solution. Diffraction properties, especially effects of the grating dimension and absorption coefficient on the diffraction efficiency are researched based on the solution in this paper. The results show that diffraction efficiency of a lossless grating increases with the higher ratio of reference beam width to object beam width. However, for strongly-coupled gratings if the absorption of medium cannot be ignored, the diffraction efficiency decreases with widening of reference beam. It is suggested that for efficiency optimization of a lossy grating the geometrical dimension of the grating should be carefully designed.