My rule; find the volume of the log in cubic feet, multiply by 1.5 and divide by 128 to determine number of cords in your log.
I've seen the tables and calculators but wanted to understand this myself. I wanted to see the log, in a stacked form.
First, the volume of a log does not equal the volume of a stack of firewood. Second, some common ground for a common question; In Vermont a cord is defined in statute (Title 9, Chapter 23 section 2651 (5)) as "the amount of wood that is contained in a space of 128 cubic feet.... when the wood is ranked and well stowed". So, when we ask "how many cords are in a log, we're assuming the log will be cut into pieces, split and ranked and well stowed. I love this, by the way. There is a almost an inference of etiquette for dealing with firewood embedded in the definition.
A cord is a measurement of volume, regardless of shape. Although we typically hear reminders that a firewood stack measured as 4' x 4' x 8' measures a cord.
The definition of a cord, therefore also assumes some amount of space between the stacked pieces of wood and rows. Also, because no piece of firewood will be the same, you will have to assume some level of error and inconsistency when measuring a cord of firewood.
Now, because the volume of a stack of firewood does not equal the volume of a log or tree we have to define some ratio or conversion factor of solid log to stacked cord.
So, I did this with my own pile of logs.
I had 17 logs of birch, maple and beech. Think of a log as a tapered cylinder. In math terms this is referred too as a truncated cone. Where the volume of a truncated cone = 1/3* Pi* h* (R^2+R*r+r^2) where
Pi = 3.14
h = log length
R = Large end radius
r = small end radius
^2 = squared
My math produced a volume of 177,962 cubic inches for the 17 logs. In terms of cords, this equates to 0.8 cords. But, remember a cord of logs does not equal a cord of stacked firewood.
Split and stacked, this measured out to 3.9' x 3.7' x 10.5' = 153 cubic feet or 1.20 cords. The conversion factor (1.2 divided by 0.8) = 1.49 log cords to real cords. Meaning, if you determine the volume of a log, in cords, multiply it by 1.49 to figure out how many cords (stacked) you will have.
While we're here, A "run" of firewood is also defined in Vermont statute as 42.67 cubic feet, or a third of a cord but is generally an antiquated term. If you have a cord of firewood measuring 4x4x8, and each piece is 16", you will have 3 rows so, the run, would be one row. A "face cord", is a confusing term, not defined in statute, typically refers to a single row of firewood that measures 4 feet high by 8 feet long. While that definition is terrible, when someone refers to a face cord, they're referring to a run, or a third of a cord. Or they should be.
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