Cantilever Deflection

What this assumes. Euler-Bernoulli beam theory: small deflections, linear elastic material, a prismatic rectangular section, and a point load at the tip of a cantilever fixed at the other end. Second moment of area I = b h^3 / 12 about the bending axis, tip deflection PL^3/(3EI), and tip bending stress 6PL/(b h^2). The L/360 limit is a serviceability rule of thumb for a member whose sag a person notices; it is a comfort criterion, not a strength criterion, and it is the one that usually bites first. Deflection scales with the cube of the length, so making a cantilever longer is enormously more expensive than making it stiffer, and orientation matters just as much: turning the section so the depth h sits along the bending axis multiplies I by (h/b)^2.