Knowing that 1 cubic foot is pretty much equal to 100 pounds or 0.05 ton, 1 ton of gravel is around 20 cubic feet.
http://wiki.answers.com/Q/What+is+the+cubic+volume...

6.65 tons of loose, dry gravel would be needed for a 140 cubic ft space. Want gravel with sand?
http://www.kgbanswers.com/howmanytonsofgravel...

Loose dry gravel has a density of about 95 pounds/cubic foot, wet is about 125 pounds/cubic foot. 1 ton = 2000 pounds. 2000 pounds / 95 pounds/cubic foot = 21 cubic foot. 21 cubic
http://answers.yahoo.com/question/index?qid=200701...

1. Multiply the total number of tons by 2,000 to convert the unit of measurement from tons to pounds. Therefore, if you have four tons of sand gravel, it would equate to 8,000 pounds
http://www.ehow.com/how_8644676_convertgravelton...
