I'll use 2 supports for each size of board and I'd like to know how far apart they should be. Perhaps someone has developed a program that tells the answer. Enter The Sagulator - it's a free online calculator for sag of shelves which is a wonderful tool exactly for these questions. This is all well and good, but what about dimension A for the shelf? Here comes the following formula: Structural Beam Bending Stress & Deflection Equations / Calculation - Cantilevered Beam with Uniform Load. This is the formula for measuring the maximal deflection at A. Comparing the two formulas ("Critical Deflection" in the first vs "Deflection at the unsupported end" in the second), one notices that the calculation is the same (Wl Alternatively, if you don't want to use wood bracing under the plywood, you might increase the number of supports so that the unsupported span is shorter. With a span of 30 CM the plywood can support about 35 KG. This would mean 3 supports for the 61 CM shelf, and 5 supports for the 122 CM shelf. Running your 2 shelf sizes, the larger shelf span (122 CM) won't hold more than about 2 KG overall without noticeably sagging. The shorter shelf (61 CM) can hold about 10 KG overall.

Inverted Metal Shelf Brackets - Etsy UK Inverted Metal Shelf Brackets - Etsy UK

I've got 122x25x1cm and 61x25x1cm plywood boards that I want to use to make shelves for books and tools. As you can see, 10 MM plywood isn't that stiff for shelf use. You can add a wood support under the shelf. Even a 1X2 (20 MM by 40 MM) attached vertically will increase the load capacity of the longer shelf to about 18 KG, and the short shelf to well over 100 KG.