Measure first
Measure finished dimensions, actual stock, openings, and site constraints before ordering material.
Shelving planning
Shelf Sag Calculator
Estimate a simply supported shelf's theoretical mid-span deflection under a uniformly distributed load. Use the result to compare spans, thicknesses, and support options during early design.
How the estimate works
Use the number as a planning starting point.
For a simply supported shelf with a uniform load, the estimate uses 5wLā“ divided by 384EI. E is the material modulus and I is the rectangular section's moment of inertia.
Check the result
Real shelf behavior depends on material direction, creep, fasteners, support stiffness, edge banding, brackets, concentrated loads, humidity, and construction details. Do not use this as a safety certification.
Plan the next step
Use the related calculators below to turn a first estimate into a material list, layout, or a more detailed project plan.
FAQ
Common Shelf Sag Calculator questions
Why do long shelves sag?
Deflection rises very quickly as span increases, and it decreases strongly with greater shelf thickness or added supports.
Can I use this for a heavy aquarium or structural load?
No. Seek a qualified professional for heavy, safety-critical, or unusual loads.
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