Shelf spacing and load calculator
Even shelf spacing for a given height, and whether the span will sag.
How it works
Divides a height into evenly spaced shelves accounting for board thickness, and estimates whether a given span will sag under load.
Sag is the failure people do not anticipate, because it is not about strength — a shelf that never breaks can still bow visibly and permanently. Deflection scales with the cube of the span, so going from 800 mm to 1,000 mm nearly doubles the sag at the same load. Doubling thickness reduces it eightfold, which is why a thicker board helps far more than a stronger one.
Material matters as much as dimensions. Particleboard and MDF creep — they deform slowly and permanently under sustained load, so a shelf that looks fine when loaded will have a visible bow in a year. Solid timber and plywood do not creep meaningfully. Books are among the heaviest domestic loads at roughly 25 to 35 kg per metre of shelf.
The usual working limit for a shelf that should look straight is a deflection of about 1/360 of the span — around 2.2 mm over 800 mm.
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Common questions
- How far apart should shelves be?
- For books, 300–350 mm suits most; paperbacks need about 230 mm and folios 400 mm or more. Divide the height evenly after subtracting board thickness.
- Why does my shelf sag?
- Span, mostly. Deflection scales with the cube of the span, so a small increase matters a lot. Particleboard and MDF also creep — deforming permanently under sustained load — while timber and plywood do not.
- How heavy are books?
- Roughly 25–35 kg per metre of shelf, which makes them among the heaviest sustained domestic loads.
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Sources
- Typical range compiled from manufacturer specifications — At One Place