Shelf Sag CheckPick the board, the span, the load and how the shelf is held. Get the sag now and after a few years, the longest span that passes, and the thickness you need, drawn on a live elevation.

Load: paperbacks about 30 kg/m, a full row of hardbacks or files 45 to 60 kg/m, folded clothes 15 to 20, display items 10. The shelf's own weight is added. A solid-wood front lip (E 10,000 MPa, top flush with the shelf) stiffens it a lot. Fully fixed ends are rare in practice, so treat that case as a best case.