Flat Plate Simply Supported Deflection and Bending Stress Calculator
Beam Deflection and Stress Formula and Calculators
Engineering Calculators
Flat Plate Deflection Calculator
Related:
Flat Plates Stress, Deflection and Reactionary Loading Equations and Calculators
Assumptions:

The plate is flat, of uniform thickness, and of homogeneous isotropic material;

the thickness is not more than about one quarterof the least transverse dimension, and the maximum deflection
is not more than about onehalf the thickness; 
all forces, loads and reactions are normal to the plane of the plate; and

the plate is nowhere stressed beyond the elastic limit. For convenience in discussion, it will be assumed further that the plane of the plate is horizontal. Beams of Uniform Cross Section, Loaded Transversely
Behavior.
The plate deflects. The middle surface (halfway between top and bottom surfaces) remains unstressed; at other points there are biaxial stresses in the plane of the plate. Straight lines in the plate that were originally vertical remain straight but become inclined; therefore the intensity of either principal stress at points on any such line is proportional to the distance from the middle surface.