there are web sites that will convert weight to area or sites that will work put how much you need such as
https://www.concretenetwork.com/conc...calculator.htm
I would have thought if you give the builders merchant the dimensions they would work it out for you.
How about a concrete base with damp proof course running through and then building on top. No need to have a wooden floor.
I have something like 15 x 10 which I dug by hand and hired a cement mixer to make up the concrete. I doubt you would need to dig very deep, 6-8 inch?
macafee2