Work out section modulus instantly with clear inputs, formula shown and shareable results.
For a solid rectangle the second moment of area is bh^3/12 and the section modulus is bh^2/6, the value that converts a bending moment straight into peak stress. A 100 x 200 mm section gives Z = 666,667 mm3 and I = 66.7 million mm4. Because depth is squared in Z and cubed in I, orientation matters enormously.
Second moment of area
I = b h^3 / 12
Section modulus
Z = b h^2 / 6 = I / (h/2)
I governs deflection and buckling; Z governs bending stress. Z is simply I divided by the distance to the extreme fibre.
Because material far from the neutral axis contributes most to I. Concentrating it in flanges gives the same strength for much less weight.