Will my shelf sag?

This estimate assumes a shelf supported at both ends, with weight spread evenly. It predicts bending right away, not long-term sag or a safe weight limit. Fasteners, joints and wall anchors need separate checks. Formula: sag = 5 × load × span³ ÷ (384 × E × I), where I = depth × thickness³ ÷ 12. E is the material stiffness. Pounds and psi stay unchanged when you switch dimension units.