How much does a cubic yard of dirt weigh?

According to this site: WikiAnswers - What is the density of dirt typical dirt weighs about 120 pounds per cubic foot, which is about 3,240 pounds per cubic yard. This is just an estimate, as the actual density can vary greatly depending on what the dirt is made of (percentage of gravel, sand, loam, etc) and moisture content.

I have 40+ years in roadbuilding and I can't improve on ebaines answer one iota. Moisture content is even covered.
The only other variable is compaction. If you were removing the earth from its original location (about 85% compacted) and mechanically compacting into a trench to 95% compaction, you would need about 10% more soil than was excavated from the original ground location.