Free calculator
Will my shelf sag?
Choose a board, enter the shelf size and what you'll put on it. We'll estimate the sag, tell you if it's acceptable, and show the longest span for every board we cut.
- MFC, MDF & plywood
- 18mm and 25mm
- Compare every board
The verdict
How we work it out
The checker treats your shelf as a beam resting on supports at both ends with the weight spread evenly along it, and adds the weight of the board itself. It uses typical stiffness figures for each board: about 2,200 N/mm² for MFC (chipboard), about 3,000 for MDF and about 7,000 for birch plywood.
- No visible sag: the shelf bends less than 1/600 of its span – about 1.3mm on an 800mm shelf. Most people won't notice it.
- Acceptable: up to 1/300 of the span. You may see a slight dip, especially over time.
- Too long: more than that. Shorten the span, add a support, or choose a stiffer board.
Chipboard and MDF slowly creep under a constant load, so a shelf that's borderline today can sag more over the years. When in doubt, go for the shorter span or the stiffer board.
Ways to stop a shelf sagging
- Fix the back edge to a batten or the cabinet back.
- Add a centre support or divider.
- Glue a hardwood lip or a second strip under the front edge.
- Use 25mm board or birch plywood for long or heavy shelves.
Common questions
How far can an 18mm MFC shelf span?
With books on a 300mm deep shelf, 18mm MFC stays free of visible sag up to about 490mm and is acceptable up to about 620mm. With light items it can go to about 850mm. Use the checker for your exact shelf.
Is MDF or plywood better for shelves?
Birch plywood is much stiffer than MDF or chipboard, so it can span roughly 40% further for the same load. MDF is a little stiffer than MFC but heavier.
How much do books weigh per metre?
A metre of paperbacks weighs around 20–25kg, and large hardbacks, records or files can be 40–50kg. The "Books" setting uses 30kg per metre and "Heavy" uses 50kg.
Will a deeper shelf sag less?
Under the same weight, yes – a deeper shelf is stiffer. But deeper shelves usually carry more, so use "Exact weight" if you know roughly what will go on it.
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