How many cords in a tree

Is there a simple method to estimate how many cords of split wood are in a downed tree or log?

Also, which would contain more wood, a cord of split wood or a cord of rounds?

OK, woodmaster gave me the idea to look for a log volume calculator but they calculate Doyle, Scribner or International units, which measure the amount of lumber that can be extracted from a log. I wasn't able to make use of these directly but it got me thinking.

A cord (4x4x8) contains 128 cubic feet. Of course, wood doesn't stack perfectly so there is not 128 cubic feet of solid wood in a cord. After some Googling, it seems reasonable to assume about 85 cubic feet of solid wood per cord.

Next, the volume of a cylinder is Pi x radius squared x length. A tree tapers as it grows so it is not a perfect cylinder; I am assuming a taper of .1 inch per linear foot. To calculate the volume of the log I calculate the volume twice -- once using the diameter at the large end, and once using the diameter at the small end and then take the average of the two. I got that idea here: http://mathforum.org/library/drmath/view/61034.html

I made a spreadsheet where I can plug in the dimensions of a log and it will estimate the value in cords. I will test it the next time I cut and stack firewood.

In any case, to get one cord of stacked wood, I calculate it would take a single log 31 feet long, with a diameter of 24 inches on one end and 20.9 inches on the other, which equals 85.5 cubic feet. And there doesn't seem to much difference between split or rounds becasue if you took that 31 foot log and cut it into four foot sections it would pretty much stack into a 4x4x8 foot area. Does that sound reasonable?