Did you get what you paid for? Or how much did you pay for the air that is included in your cord of firewood.
There is no use in real life for this unless you are trying to calculate the amount of BTU's for your cord of firewood. And just to point out that the sales man that sold you your firewood in addition sold you some air at 100% profit. You buy a cord of firewood at a certain price and you come home and stack it up neatly and measure the wood pile and calculate what you have. What was the actual price you paid for the wood and how much did you pay for the air in your cord of firewood?
It is said that a cord of firewood is equal to 128 cubic feet, which occupies an area of 8' length 4' height and a depth of 4'. Depending on how neatly the wood is stacked there obviously is a certain amount of air in that area in addition to the wood. You would be lucky if that cord of wood you just bought by those measurements have a volume of 90 cubic feet. In addition to that, if you bought your cord of firewood in green state there will be a certain amount of shrinkage to your pile, further reducing the cubic foot volume of your cord of wood.
Since the shrinkage of wood varies depending on the species of the wood 5% is used as the factor for shrinkage. (It could be a bit less, or much more depending on the wood species.)
The packing factor will determine how much air there is in your pile of wood. 20% is not uncommon and it could be higher or a bit less.