Work out shelf load capacity instantly with clear inputs, formula shown and shareable results.
Shelf capacity is governed by deflection, not strength: a shelf that sags 10 mm looks broken long before it fails. Deflection scales with span cubed and inversely with thickness cubed, so halving the span multiplies capacity eightfold — far more effective than a thicker board.
Shelf deflection
allowable load = (span/200) x 384 E I / (5 x span^3), with I = depth x thickness^3 / 12
It is the point at which sag becomes visually obvious. Structural failure happens much later, but nobody accepts a visibly bowed shelf.
A centre support. Halving the span increases capacity by eight times, whereas doubling thickness only achieves the same eight times at far greater cost and weight.