You're calculating the weight of a metric ton in pounds, which is an odd thing to do. The metric ton is 1,000 kilograms. The short ton (what is generally just called a "ton" in the U.S.) is 2,000 pounds. If you consistently use the appropriate unit system (metric or imperial), calculating the weight of either type of ton is a snap. Since the D&D books use pounds, I would assume they are also using short tons and go with that.
(There is also a "long ton," which is defined as exactly 2,240 pounds, because... uh... look over there. This is the ton to use if a) you have to deal with it in the real world for some reason, or b) you think the British currency system pre-decimalization was too simple.)
And then there is gross tonnage, per
@Flamestrike's link, which is a volume-based measure that requires logarithms. If that appeals to you, you may wish to consider giving up D&D and looking into GURPS.