Wall shelves require a bit of planning before loading up your favorite dishes or plants. A shelf span is the distance between supports for the shelf. This span distance will vary based on the strength of the material of the shelf. In theory, the stronger the shelf the greater the distance you can put between wall supports. Shelf spacing will depend on preference and what you will store on the shelves. It’s important to fasten the shelf supports to wall studs whenever possible for maximum support.
The span limit will change depending on the material of the shelf as well as the load it will carry. For the following recommendations it’s assumed the shelf will support a full load of standard-sized hardback books.
Plywood. Shelves should be ¾ inch thick or thicker. The longest recommended span is 36 inches between wall supports.
Lumber. The longest recommended span between walls supports for 1×10 lumber is 24 inches. If the lumber is 1×12 then it is 28 inches.
Particleboard. Single layer shelving made of particleboard should have a thickness of ⅝ inch. The recommended span for this would be 24 inches. If you have a thicker particleboard of ¾ inch then you can span wall supports by 28 inches.
The vertical spacing between shelves will ultimately be determined by the objects it will hold. Here’s a general clear shelf height depending on specific items.
Paperback books: 8 inches
Hardback books: 11 inches
Large hardback catalogs: 14 inches
Magazines: 14 inches
DVDs: 8 inches
Source: the Spruce