Tons a trash bin holds- Does this make sense?

It depends on whether the food is primarily liquid or solid. You should to calculate the specific gravity of the material to determine the weight of the material. Your calculations above appear to have the specific gravity of water which is a pretty good baseline. And your calculations are accurate. I can only assume you are trying to calculate the charges associated with disposal to a municipal landfill. Send me an email if you need some assistance in calculating rates.