How Solid is a Cord of Wood?

The author's experiment.

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.

 
Discussion
  1. John D Fox Jr → in Fairbanks, AK
    Oct 13, 2014

    Brian,

    I enjoyed your article on “How solid is a cord of wood”.  Using the fencing was a clever way to sample the face of the cord.  I’ve always accepted the textbook claims that a cord is a space of 128 cubic feet occupied by solid wood of between 80 and 90 cubic feet (see Avery’s “Forest Measurements”).  Tracing back where that range comes from is as hard as finding the source of any “conventional wisdom”.

    A couple of interesting references that might be available on the internet are a 1950 report of cubic feet per cord for pulpwood in the Pacific Northwest (by Worthington & Twerdal). Mill log scalars measured end diameters of 8-foot logs in a cord and calculated cubic volume per piece, and then per cord.

    Even more interesting is the discussion and data provided in Carl Schenck’s 1905 book: Forest Utilization, Mensuration.  The studies he reported (page 13-15) did deal mostly with fuel wood and used the method of water displacement to determine the volume of each round or split piece.  He reported several conditional results ranging from 102.4 cubic feet for “First class split wood”, to 25-51 cubic feet per cord for sticks.

    So, what does it all mean? The answer seems to be “it all depends”!  You might have the right approach… make your own measurements based on the piece size, length, number of rows, overall dimensions, etc. that best describe your wood.  I usually use 85 cubic feet per cord for planning purposes. Better to underestimate the cubic feet per cord and end up with extra wood at the end of the season than overestimate it and run out!  Sounds like once you make your own measurements it’s up to someone else to prove you wrong!

    Cheers,
    John Fox

  2. Rodger D Tucker → in USA
    Oct 25, 2014

    How about taking 8’ long logs,take the required measurements to determine how many cu.ft are in that log. Then do another & another until it comes as close to 128cu.ft as you can get it. Done right? Not yet. Now take & cut logs & split to firewood size. Now you can stack it up starting with the 4’X4’X8’ area. Now, how much of that pile of wood will not fit into your 4’X4’X8’ area?

  3. Ted Cady → in Warwick, MA
    Feb 05, 2016

    The procedure discussed is faulty in two ways.  A cord of wood is defined as 4’x4’x8.’  When you cut it to firewood length it will shrink significantly because the cutting “straightens” out the pieces.  The second error is that the original measure was used for pulp wood which was not split. 

    The Woodburners Encyclobedia claims a 25% reduction in size when 4’ wood is cut “to length” because of closer stacking and sawdust loss. It also says that maximum amount of wood is obtained when smaller pieces are fitted in between larger ones in the pile.  They claim the volume of a cord varies between 60 and 100 cubic feet depending on the shape of pieces and how they are stacked.

    I note that my wood piles settle over time.  I do not know if it is a result of shrinking from moisture loss or just settling in with time as the pieces of wood get to know each other. 

    Years ago I read a Forest Service Report that stated that splitting firewood some times fluffed up the pile and sometimes caused it to shrink. 

    We all know that branch wood does not tally up the way trunk wood does.  Also, it seems that trees with thick, rough bark do not stack as tightly. 

    Ted Cady

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