Find the formula for the stress in the given situation. Examples include s = -WL / 4Z, for the stress at the center of constant cross-section, as in the center of a square beam. Equations you find should include definitions of the variable names used, and in this example, s is the stress, W is the load, L is the length of the beam, and Z is the section modulus of the cross-section of the beam.
Calculate the moment of inertia or section modulus. These are similar concepts that measure how easily a beam bends based on its cross-section and equations involving the stress in a beam will require one or the other. For a square beam, the moment of inertia, I, is (a^2 - b^2) / 12, and the section modulus, Z, is (a^4 - b^4) / 6a, where a is the width of the beam, and b is the width of the inside of the square tubing ( = 0 if the beam is solid).
Find the Young's modulus, E, of the material of the beam, which tells you how easily a material deforms under stress. There are many charts available for this purpose and engineering students can usually find them on or near the inside cover of their textbooks or online. Sometimes, the Young's modulus may be given to you in the problem itself.
Plug in all relevant values for the variables in the equation and calculate the answer.