Check shelf sag from span, material, thickness, and load before you cut or buy material.
Measure the clear span between supports, the front-to-back depth, and the actual shelf thickness.
Use modulus of elasticity from a data sheet when you have one.
Use the total expected shelf load unless you choose load per length.
This is a furniture-scale estimate for a shelf supported at both ends. It is not a structural beam approval.
A common visible-sag target is 0.02 in per foot or about 1.7 mm per meter.
Target sag: 1.5 mm
Long-term sag is estimated as 1.5x initial sag for shelves that stay loaded.
Simple shelf with even total load: sag = 5 x W x L^3 / (384 x E x I).
I = depth x thickness^3 / 12.
Saved shelf checks stay in this browser only.
Shelf sag depends most on span, thickness, material stiffness, and load. Use this calculator as a planning check before cutting parts or buying boards.
A 36 in span, 12 in deep, 3/4 in thick shelf with a 100 lb even load sags about 0.096 in at first, above a 0.06 in target.
A 24 in span, 10 in deep, 3/4 in shelf with a 60 lb even load sags about 0.026 in, under a 0.04 in target.
A 36 in span with 1.5 in thickness sags about 0.012 in under the same 100 lb even load because thickness has a large effect.
Measure the clear span between supports, the front-to-back depth, and actual thickness. Nominal lumber size can give the wrong answer.
Use even total load for books or dishes across the shelf. Use center load when one heavy item sits near the middle.
The target is a visual comfort limit, not a safety rule. Use a smaller target for visible furniture shelves.
If sag is high, shorten the span, add a center support, add a front edge, or use a thicker shelf.
Deflection changes with thickness cubed, so 0.75 in and 1 in are very different in the math.
Wood stiffness varies by species, moisture, grade, and board. Use a custom data-sheet value when accuracy matters.
Books, dishes, and stored tools stay on the shelf. Long-term sag can matter more than the first-day number.
This calculator is for shelves and cabinets. It does not check building code, fasteners, anchors, or structural safety.
A thicker shelf can reduce sag much more than a small change in wood species.
Splitting one long span into two shorter spans can make a visible difference.
If you are using plywood, MDF, or particleboard, use the manufacturer's stiffness value when available.
Save a few span and thickness options on this device, then compare them before you cut.