Sagulator
Estimate how much a wood shelf will deflect under a uniform load (books, dishes, stereo gear) including its own weight. A common rule-of-thumb cutoff is a deflection ratio of about 1/180 of the span for bookshelves; tighter (1/240–1/360) for fine cabinetry.
Max deflection: 0.035" (≈ L/1040)
Includes shelf self-weight of 4.9 lb.
The formula
δ = 5 w L⁴ / (384 E I) for a simply-supported shelf, δ = w L⁴ / (384 E I) when both ends are fixed. I = b h³ / 12 with b = depth and h = thickness; w is total uniform load per unit length (applied + self-weight); E is the species' modulus of elasticity.