Incredible tool for DIY workspace / shelving-needer / amateur wood-muckers:
The Sagulator helps you design shelves by calculating shelf sag (deflection) given type of shelf material, shelf load, load distribution, dimensions, and method of attachment. You can also specify an edging strip to further stiffen the shelf. See the notes below for usage tips.
Cannot testify as to its accuracy as I am still searching for second-hand plywood to use a desk, and probably not find any before the end of this website. Godspeed