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span ÷ sag ratio
Shelf Sag Calculator
Will your shelf sag? Enter the span, depth, thickness, material and the load it carries, and see how far it will deflect in the middle — before you cut anything.
Your shelf
Updates live
Units
Span is the unsupported distance between supports. Load is the total weight spread along the shelf (books ≈ 10–20 lb per running foot).
Result
Estimated sag at center0"
deflection at mid-span
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sag limit (L/360)
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stiffness (E, psi)
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load on shelf
How to read this
The shelf is modeled as a beam supported at both ends carrying a uniform load. Deflection grows with the cube of the span and shrinks with the cube of the thickness — so a slightly thicker shelf or a shorter span helps far more than you'd expect. A result tighter than L/360 is stiff; up to L/180 is usually acceptable for shelving; beyond that the sag becomes obvious. Values use typical published modulus figures, so treat them as a strong estimate.
How much shelf sag is acceptable?
Keep deflection under the span ÷ 360 (L/360) to be safe; up to L/180 is commonly tolerated for shelves. Past that it looks bowed.
How do I stop a shelf from sagging?
Shorten the span or add a center support, make it thicker, add a front edge/apron, or switch from MDF/particleboard to plywood or hardwood.
Is the shelf sag calculator free?
Yes — completely free, no signup, and it runs entirely in your browser.
Planning the whole build?
Estimate material with our board foot calculator and lay out cuts with the cut list optimizer.