Finite element models for plate bending problems are constructed on the basis of approximations of moments fields. The bending and twisting moments are approximated in the finite element area by piecewise constant functions. The solution is based on the functional of the additional energy. Algebraic equations of equilibrium of nodes of grid of finite elements are formed using the principle of possible displacements and are included in the functional with the help of Lagrange multipliers. The necessary expressions for rectangular and triangular finite elements are obtained. Calculations of square clamped and hinged-supported plates on the action of uniformly distributed load are performed. Comparison of the obtained results with the results by the finite element method calculations in displacements is presented. It is shown that the presented method of calculating bent plates by the finite element method in stresses has the property of convergence from above. The displacements obtained by this method converge to the exact values from above, while the values of the moments is determined with reserve. When the grid of finite elements is crushed, the difference of the two solutions, in stresses and in displacements, decreases monotonically and the accuracy of the obtained results can be estimated from the value of this difference.