Generally speaking I use the following rule of thumb, 1/2 yard per square. A 21 square roof with two layers would follow this formula: .5 x 21 x 2 = 21 yards. This .5 yards per square is on the high side and will leave you room for cut offs, packaging, felt, etc... If I were ordering a dumpster I would order a 20 yarder and would have room to spare for neighbor's donations.
Sometimes the savings that comes from doing it yourself can be blown away with one mistake.
The information found in this post is not to be considered legal advice. All information should be considered relative, not specific. Never attempt any repairs you are not comfortable with. Always maintain safety! The author of this post takes no responsibility for any losses that occur. Use at your own risk.