I have a shelf that is about 4 feet long 2 feet wide and 1 1/2 inch thick. I would like to place an item on it that weighs about 90 pounds. I want to know if it will hold that and I don't know how to calculate it.
There is a cool site for this: The "sagulator" for computing this. It's intended more for freespan shelves (like a book case), but you might be able to use it to answer your question. Hope it meets your needs. Ed makes a good point about support as well. With that weight it will need to be fastened to studs with multiple brackets.
Edit: Forgot the link, here it is: https://www.woodbin.com/calcs/sagulator/