For timber, length x breadth x thickness divided by 1728 gives you cu ft. Each species has it's own density therefore weight Google dry weight per cu ft of say yellow pine then google weight per cu ft ally bobs your uncle. If you are thinking of horse box floor ? Then you have to use a mahogany called keroin, all other timbers fizz and bubble when soaked in horse pee and give out after a few hissy fits. Kerion is gut bustingly heavy, A clue to it's popularity in horse box floors. Ply wood is not an option unless covered in armored plating. Hope that helps.
Ply (esp phenolic coated with edges sealed) works perfectly well, just has to be considered a consumable on a two or three year view. A bit longer if you're lucky and yr horses cross their legs.