Let me just say that before start getting too riled up here, US pot sizes are non-standard, and just because something is sold as a certain volume, does not mean it actually is that volume. You may be able to calculate what the actual volume of something is, but that does not mean that it will be sold under that size, not to mention that it all varies by manufacturer.
Check out this link for a bit more info
Understanding Container Sizes and Their Volumes - Container Gardening Forum - GardenWeb