The shelf
The span is the clear distance between the two supports, not the whole shelf. Thickness counts most, because the stiffness goes up with the cube of it.
The load
The total weight it will carry, spread evenly. A shelf of books runs about 20 to 25 kg per meter. The shelf's own weight is added for you.
Fill in the shelf above.
The sag is for an even load on a shelf resting on a support at each end. A point load in the middle, or a shelf fixed into the wall along its back, behaves differently. Treat the number as a close guide, not a guarantee, and leave yourself some room.
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Watch the Short
Will your shelf bow? Particleboard vs plywood
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A shelf that looks fine empty can bow once it is full, and a bowed shelf never quite comes back. The good news is you can tell before you build it. Put in the span, the depth, the thickness and the material, say how much it will hold, and this works out how far the middle will drop.
What makes the difference
Thickness. This is the big one. A shelf's stiffness goes up with the cube of its thickness, so going from 18 to 25 mm sags about a third as much, not a quarter less. If a shelf is too bouncy, thickness is the first thing to add.
Span. The distance between supports, cubed as well. A shelf twice as long sags eight times as much under the same load. A support in the middle, halving the span, is the strongest fix there is.
Material. Solid wood and plywood are stiff. MDF is about a third to a half as stiff, and particleboard and melamine are worse, so the same shelf in MDF sags two to three times as much. For anything heavy, plywood or solid wood earns its place.
Depth, front to back. It helps, but only in proportion, so it matters far less than thickness. A deeper shelf is a little stiffer, not a lot.
How much sag is too much
The usual structural limit is the span divided by 360, and the calculator shows it next to your result. That is the point where sag starts to be a problem, not where you first see it. For a shelf you look at every day, aiming for half that, span over 720, keeps it looking dead flat. The verdict above tells you which side of the line you are on.
Ways to stiffen a shelf
Add thickness. Shorten the span with a middle support. Switch a weak material for a stiffer one. Or glue a solid wood edge along the front, an inch or two deep standing on edge, which stiffens a plywood or MDF shelf far more than its size suggests, because that edge is deep where it counts.
More MakerKiln tools
- sheet planCut List OptimizerIt figures out how much plywood to buy and shows you the layout to cut from.→
- pin holesShelf Pin Jig GeneratorIt drills shelf pin holes on 32 mm spacing at the same setback every time, so adjustable shelves sit level.→
- drill guideDowel Jig GeneratorIt makes a drilling guide for your board and dowel, with the holes on the center of the edge.→
- price itHandmade Pricing CalculatorMaterials, your time, running costs and sales fees - the price that leaves a real profit.→