Reference

How much weight can a shelf hold?

The short version: it depends on the shelf's width, material, and thickness, and the risk is sagging, not snapping. As a guide, an 18 mm MDF shelf up to about 70 cm wide comfortably holds a normal row of books; wider shelves, or heavy loads, need a stronger material, a thicker board, or a center support.

What "light", "medium", and "heavy" mean

Load is measured per metre of shelf. Here is what each level looks like:

Load level	Weight per metre	Example
Light	about 10 kg/m	decorative items, a few light objects
Medium	about 30 kg/m	a normal row of books
Heavy	about 60 kg/m	packed books, records, tools

So an 80 cm shelf holds roughly 8 kg at a light load, 24 kg at medium, and 48 kg at heavy, as long as the shelf is stiff enough not to sag.

Why width and material decide the limit

A shelf rarely breaks; it bows over time. The wider the shelf and the weaker the material, the less weight it holds before it sags.

Plywood and pine are about twice as stiff as MDF, so they hold more weight for the same width. Compare MDF, plywood and pine
Keep an unsupported 18 mm MDF shelf under about 70 cm for everyday loads; beyond that, add a center support or use a stronger board. See standard shelf dimensions

The rule of thumb

If your shelf is wide or the load is heavy, do one of these: shorten the span, add a center support, use plywood or pine, or move to a 25 mm board. Any one of them buys you more weight before the shelf sags.

How Kamba works it out

Kamba does not guess. For your exact width, material, thickness, and expected load, it calculates the deflection and keeps every shelf within safe limits, well under a visible sag. If your design would sag, Kamba tells you and recommends a fix, so the shelf you build holds what you put on it.