Structural Beam Deflection and Stress Formula and Calculation: The follow web pages contain engineering design calculators will determine the amount of deflection a beam of know cross section geometry will deflect under the specified load and distribution. Please note that all of these calculators use the section modulus of the geometry cross section of the beam. You will need to determine the moment of inertia of the cross section and the distance from the neutral axis to the edge of your geometry.