Ideally a cord of firewood would be a pile of wood 8' long with a height of 4' and a depth of 4'.
But how many cords of fire wood you actually have in those measurements depends on how the pile is stacked.
Air comprises much of the gross volume of a pile. The ratio of wood volume to the total pile volume is the packing ratio. From destructive measurements of 17 piles, Hardy (1996) found that packing ratio ranges from 0.06 to 0.26.
According to the above sampling of 17 piles the amount of airspace in your stacked wood pile could be anything from 6% to 26%.