Shelf Sag Calculator
A front lip beats a thicker board. Depth is cubed.
Work out Shelf Sag. A front lip beats a thicker board. Depth is cubed. Every rate is an input, not an assertion.
Deflection at mid-span
1.3 mm
Within the span/360 limit of 2.22 mm · 1 in 614
Stiffness goes with thickness cubed and sag with span cubed at a fixed load. Doubling the thickness cuts the sag eightfold; stretching the span 25% raises it roughly twofold. Span is what to attack first. A lip glued to the front edge beats a thicker board for the same material, because depth is what the cube applies to. A 30 mm lip on an 18 mm shelf is stiffer than a 40 mm solid board and uses a fraction of the timber. Particleboard is the trap. It looks like a shelf and has about a fifth of oak's stiffness, which is why flat-pack shelving sags within a year of being loaded with books it was never sized for. Melamine veneer contributes nothing structurally.
How the Shelf Sag Calculator works
Deflection at mid-span from span, thickness, material and load, against the span/360 limit that reads as flat. Reports the longest acceptable span and the thickness that would fix it.
Also known as: how far can a shelf span · will my shelf sag under books · sagulator alternative · shelf thickness for a 1 metre span
Frequently asked questions
How far can a shelf span?
It depends on material and load, but roughly 700 mm for 18 mm particleboard under books, 900 mm for plywood and over a metre for solid hardwood. Beyond that, sag becomes visible.
What counts as too much sag?
Span divided by 360 is the usual joinery limit. Below that it reads as flat; above it, the eye picks up the curve and the shelf looks like it is failing even when it is not.
Does a thicker shelf help?
Enormously — stiffness goes with thickness cubed, so doubling it cuts sag eightfold. But a lip glued to the front edge exploits the same cube for a fraction of the timber.
Why does particleboard sag so much?
It has about a fifth of oak's stiffness. Melamine veneer adds nothing structurally, which is why flat-pack shelving sags within a year under books it was never sized for.
Does fixing both ends help?
By a factor of five, if genuinely rigid. In practice most shelf fixings are somewhere between simply supported and fixed, so treat the simply supported figure as the safe one.
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