According to this document
at the US NIST, the average density of dirt is 120 lbs per cubic foot.
If you multiply this density factor by the number of cubic feet, you will get your weight in pounds. The cubic feet is determined by multiplying the length, width and depth of your ditch together.
To convert from pounds to tons, I think you divide by 2240 pounds in a ton. If you are using American tons instead of imperial (long) tons, I think the ratio is 2000 instead.
Here in Canada we use metric where everything is a power of 10. It's a lot simpler just moving the decimal.
Stephen La Rocque.>