.. want to know the volume of your tank and how much weight when loaded?? .. this one will get you pretty close .. and you can adjust the fill depth ..
.. and if you are using a dipstick, or need to use one, check out the little yellow box to the right, it will give you the increment measurements for gallons of fill ..
.. you can also find a calculator on that sight for cylinders .. in case you are one of the folks using plastic drums ...
.. good site for general construction calculators too .. site has imperial and metric calculators ..