How much dirt can a one gallon pot hold?
Not really. While a pound of lead is the same as a pound of feathers, apparently the horticultural industry does not believe the same is true for gallons. One trade gallon is roughly .71 U.S. gallons. source http://en.wikipedia.org/wiki/Trade_gallon
But it can also vary by manufacturer. here is an excellent link explaining container sizes and volumes better than i can