Most of us have been trained to picture a cord of wood as a neatly stacked pile measuring 4x4x8 feet. But how much of that 128-cubic-foot rectangle is wood and how much is air?
Searching on the internet, I found the consensus to be that one cord of wood contains about 85 cubic feet of solid wood, which means that 43 cubic feet of that must not be wood. That breaks down to roughly two-thirds wood and one-third air. But I’ve stacked a lot of wood, and my instincts told me that couldn’t possibly be right. To visualize what a stack that airy would look like, I took my son’s Lego set and made a wall that was exactly one-third air. There was no way I could stack a row of wood with that much air in it even if I tried.
But Legos aren’t firewood. To figure out how much air was in a row of firewood, I came up with the idea of stapling a piece of garden fence to my stack of wood and counting the number of times I found air versus wood at each corner of the 1-inch x 3-inch rectangles. With enough samples, I would have a good approximation of how much wood and how much air there was. The area of fence that I counted had 200 corners, and I found wood 171 times. So, in my case, a well-stacked row of wood is 86 percent solid, which in a perfectly stacked row would amount to about 110 cubic feet of wood.
But my row is not a perfectly stacked cord, nor is each piece cut to exactly the same length. I typically cut my firewood to 16-inch lengths, measured with a stick of wood and marked with an axe, hardly a precise method. Sampling my stacks, I found that my presumed 16-inch pieces of wood ran between 15 and 17 inches, averaging 15.5. (Nor is every piece cut square on the ends.) My pieces are short by an average of 3.1 percent, so my stacked “cord” is now down to around 106 cubic feet of wood. There’s undoubtedly more loss in other places, like the bark itself. I’m happy when I come across black locust, but there certainly is a lot of air space contained in its deeply furrowed bark.
So where did this consensus figure that a stacked cord is two-thirds wood come from? I was able to contact the owner of one of the websites that published that information. He informed me that his figure wasn’t based on measuring a carefully stacked cord but rather on a study that measured the average weight of wood that was delivered to a typical consumer. This is quite different than the way I was looking at it. In that context, how would you know if the cord was really a cord to begin with? I found tables online that gave both the weight per cubic foot and the weight per cord of several firewood species, and some quick math indicated that there’s roughly 60 percent solid wood in a cord, though it’s not clear what assumptions they made about size and stacking.
Even simple science provides more questions than answers. I think I was able to prove, though, that an honest cord will be, at best, about 82 percent solid wood when stacked. Then that cord will shrink as it dries – 6 percent or more. Is it no longer a cord at that point, or is it a cord with more air space? More questions to ponder, but the good thing is that all of the BTUs are still there.