I've never taken firewood BTU charts too seriously since: (1) where does the info come from? and (2) different charts say different things. Directly testing the amount of heat (BTUs) produced when burning wood is not practical, so you need an indirect approach. The best indirect approach is using wood density (dry weight per a given volume) as a proxy for potential BTUs, since one pound of any type of wood at a given moisture content contains roughly the same amount of potential heat (at least in theory, caveats discussed below). Wood density (pounds per square foot @ 12% moisture content) has been studied scientifically for nearly all types of wood on planet earth. The USDA has funded such studies, and the results have been published in books such as Hardwoods of North America by Henry Alden, among other places. The Wood Database website — a site geared towards woodworkers — gives the technical characteristics for many types of wood, including density (pounds per square foot @ 12% MC), gathered from USDA publications and other reliable sources. My calculations below are based on density figures from the Wood Database website. BTUs per Cord Calculation using Wood Density First, a pound of wood @ 12% moisture content contains about 7,640 BTUs in potential combustion energy. Second, a cord of firewood contains about 85 cubic feet of solid wood. (A stacked cord is 4' x 8' x 8', or 128 cubic feet, but that includes the space between splits [air]. 85 cubic feet is the volume of the actual wood, assuming 2/3 wood and 1/3 air.) The formula for BTUs per cord for a given type of wood is: (pounds per cord) x (BTUs per pound). Pounds per cord is: 85 x (pounds per square foot @ 12% M.C.) BTUs per pound at 12% MC is a constant 7,640 So the formula becomes: 85 x pounds per cubic foot x 7,640 = BTUs per cord You can first multiple 85 x 7,640 = 649,400 649,400 becomes the magic number. Multiply that number by the pound per cubic foot for a given species @ 12% MC, and you get the BTUs per cord for that species. I ran this calculation a bunch of times, primarily for species that grow in or around Michigan, since that's where I live. The final number is millions of BTUs per cord based on the stated lb/ft3 @ 12% MC Osage orange 54 lbs lb/ft3 = 35.1 Pignut hickory 52 lb/ft3 = 33.8 Apple 52 lb/ft3 = 33.8 Mockernut hickory 51 lb/ft3 = 33.1 Shagbark hickory 50 lb/ft3 = 32.5 Hophornbeam 49 lb/ft3 = 31.8 Swamp chestnut oak 49 lb/ft3 = 31.8 Swamp white oak 48 lb/ft3 = 31.2 Shellbark hickory 48 lb/ft3 = 31.2 Black locust 48 lb/ft3 = 31.2 Honey locust 47 lb/ft3 = 30.2 White oak 47 lb/ft3 = 30.2 Post oak 47 lb/ft3 = 30.2 Bitternut Hickory 46 lb/ft3 = 29.9 Burr oak 45 lb/ft3 = 29.2 Black oak 45 lb/ft3 = 29.2 American beech 45 lb/ft3 = 29.2 Sugar maple 44.0 lb/ft3 = 28.6 Pin oak 44.0 lb/ft3 = 28.6 Red oak 43.8 lb/ft3 = 28.4 Mulberry 43 lb/ft3 = 27.9 White ash 42 lb/ft3 = 27.3 Black maple 40.0 lbs/ft3 = 26.0 Norway maple 40.3 lb/ft3 = 26.2 Teak 40.1 lb/ft3 = 26 Black walnut 38 lb/ft3 = 24.7 Red maple 38 lb/ft3 = 24.7 Red Elm 38 lb/ft3 = 24.7 Tree of Heaven (Ailanthus) 37.1 lb/ft3 = 24.1 River birch 37 lb/ft3 = 24 Hackberry 37 lb/ft3 = 24 Black cherry 35 lb/ft3 = 22.7 American elm 35 lb/ft3 = 22.7 Red pine 34 lb/ft3 = 22.1 Black Tupelo (Sour gum) 34 lb/ft3 = 22.1 Eastern red cedar 33 lb/ft3 = 21.4 Douglas fir 32 lb/ft3 = 20.8 Box elder 30.2 lb/ft3 = 19.6 Eastern cottonwood 28 lb/ft3 = 18.2 Black spruce 28 lb/ft3 = 18.2 Red spruce 27 lb/ft3 = 18.2 White spruce 27 lb/ft3 = 18.2 Quaking aspen 26 lb/ft3 = 16.9 Black willow 26 lb/ft3 = 16.9 Basswood 26 lb/ft3 = 16.9 Eastern white pine 25 lb/ft3 = 16.3 Balsam fir 25 lb/ft3 = 16.3 Norway spruce 25 lb/ft3 = 16.3 White willow 25 lb/ft3 = 16.3 Northern white cedar 22 lb/ft3 = 14.3 Balsa 9 lb/ft3 = 5.8 Notes / Caveats (90%+ of people will want to skip this part as I wander deep into the weeds -- and please, correct me if you think I'm wrong) 1. Wood with combustible sap (softwoods like pine / spruce) and hardwoods that contain natural oils in the wood or bark (teak, hickory, cherry, etc) have slightly more BTUs per pound since sap and oil are more energy dense than the cellulose / lignin in wood. This probably makes little difference for wood like cherry (oily bark, but not very oily wood), but could potentially make a big difference in the case of very sappy softwoods. Nevertheless, sap is highly volatile and evaporates quickly in a hot stove. Most stoves are not well-designed to efficiently burn a sudden burst of highly combustible gasses (vaporized sap), so the extra BTUs that come from sappy softwoods may not translate into a big boost in actual heat output, and certainly not a sustained increase over the entire burn cycle. 2. Don't get too caught up on the absolute number of BTUs per cord in the above calculation. I know that they look high compared to most other charts, but that doesn't matter. Where this chart should be more accurate than other charts is the relative ranking, which is ultimately what is useful. Instead of using 7,640 btu per pound in the calculations, you could use something like 5,000 to 6,000 btu per pound, which is probably a more accurate approximation of the amount of heat that will be transferred by the stove into the heated space. In that case, all of the above calculations of BTUs per cord could be reduce by about 20-30%. The relative ranking would remain the same. 3. My calculations use 12% MC because the available weight data (from USDA studies and other technical studies) is based on 12% MC. Few of us season wood down to 12% MC. Firewood that has a moisture content of > 12% will have slightly less BTU's per cord, since heat is used to evaporate the moisture from the wood during combustion. The difference is not that great, however, until you cross a certain threshold (around >20% MC?) at which point combustion will either become incomplete, or you will require a ton of excess air flowing through the stove to maintain complete combustion. Additional ramblings about stove efficiency and the moisture content of wood... Under an incomplete combustion scenario, heat is lost not only to the evaporation of water from the wood, but also from combustible gasses going up the chimney without being burned. When you combine the two losses (less heat from incomplete combustion, and lots of that heat being used to evaporate water) overall heat output can become dismal when burning wood with 25-30% MC. You can sometimes overcome incomplete combustion by allowing lots of air to flow through the stove, but you will increase stack loss, or the amount of heat going up the chimney. Wood stove design, draft strength, split size, among other factors, will all affect the moisture content range at which a stove obtains good combustion efficiency and low stack loss -- the ideal situation. In my experience (with my stoves / my chimneys / my splits sizes / etc, ymmv) around 14% to 18% MC seems to allow for high combustion efficiency and low stack loss. I can quickly assess this by looking at the relationship between the stove top temperature and the temperature of the stove pipe. Ideally, I want a really hot stove with a relatively cool chimney. In other words, lots of heat being kept in the room, and not a ton of it going up the chimney. Both of my stoves / chimneys have a strong draft, so I have to greatly restrict the amount of air flowing into the stove to reduce stack loss. Only very dry wood (14%-18% MC or so) will permit efficient combustion with very little air under moderate to strong drafting conditions. When wood is too dry (yes, that is possible), it is hard to maintain good combustion efficiency, since the wood will volatilize too fast, leading to incomplete combustion if air flow is highly restricted. If more air is let in to avoid incomplete combustion, then the stove can overfire. I can overcome this is part by blowing a ton of air over the surface of the stove. I think this could also be addressed with stove design. To efficiently burn extremely dry wood, I believe that the stove would need to allow for good air / fuel mixing with low rates of air flow. Imagine numerous tiny secondary air holes, with a small amount of pressurized air flowing into them, or better yet, an oxygen concentrator feeding the stove with a small amount of pressurized, oxygen enriched air. In reality, stoves are designed to efficiently burn wood in the 15-20% MC range, which makes sense. A stove designed to efficiently burn ultra dry wood would require ultra dry wood, which is probably not a good trade off for most people. Rambling finished.
Thanks for your work Jonathan Y. Cut a full truck load of Shagbark and red oak earlier this morning. Most of the hickory I cut is of the butternut and mockernut variety. White oak is my all around favorite. Nice to see most of what I cut is in the top BTU category.
A few things I noticed... These calculations confirm that Norway Maple is a premium firewood that is almost as good as oak. That has been my experience as well. I love Norway Maple. It is more similar to sugar maple than it is to the soft maples. The calculations also show that red elm and red maple are above average, which has also been my experience. Is is easy to confuse red elm with american elm, although red elm has almost 10% more potential heat. Same with red maple, which is often grouped together with silver maple (both are "soft maple."). Red maple is much better firewood than silver maple, although I don't mind burning either of them. I was surprised to see where Tree of Heaven falls in the lineup, since it is often looked down upon. Also surprised at the variation among the different types of hickory. I've cut and burned them all, and they all seem about the same to me.
Wow! My hat’s off to you. Impressive work and very well thought out. You sir, are a wood nerd among wood nerds
I wish someone would do a btu test on standing dead Siberian elm. Has been my experience that it punches well above its weight. Even seems better than the little bit of oak that I have been able to get ahold of. At least equal to mulberry, which I get quite a bit of. Standing dead is significantly better than if it is processed green…I’m going to say at least 30% better. Not sure what makes it better, but it is mostly what I go after around here. Splits pretty easy as well. Luckily there is lots to be had in this area. Most landowners are more than happy to let people cut in the creek bottoms and old farmsteads.
I like the scientific process application. Yeah that's some variables and I for one think that most BTU charts are fairly off.
I cut small and medium size (6-12" diameter trunk) standing dead red elm from my forest. It is super hard, heavy, and dense. Every bit as much heat in the stuff as oak. These trees spend most if not all of their lives as understory trees in an oak-hickory forest that is dominated by 100' oaks and hickories. I've cut 6" diameter dead red elms that were 50 years old, since that's how slow they grow in the shade. Big elms that grow out in the open are completely different, at least around here -- the wood is not nearly as dense, although they still make good firewood.
If you have a fairly straight and round piece of the dead siberian elm and a moisture meter, we can do the btu calculation using the dimensions and weight of the log. For example, a 6" diameter log that is 18" long is 509 cubic inches, or about 0.295 cubic feet. Let's say such a log weighs 10 pounds and has 20% moisture content. At 20% moisture content, wood has about 7000 btu per pound. A cubic foot would weigh 33.9 pounds and have 237,300 btus, and a cord (85 cubic feet) would have 20.2 million btus. I am just making up numbers to show how you can use the formulas from my original post to calculate the btu content of a cord of any type of wood if you can measure a piece of the wood fairly precisely, weigh it, and test its moisture content. If you want to get real precise, you could calculate the volume of a log using a 5 gallon bucket and water. Add the log to the bucket and fill with water. Remove the log and then measure how much water is left in the bucket. One gallon is about 0.134 cubic feet, so if the log took up two gallons of space, you know the log is 0.268 cubic feet. That would be a more precise method and measuring a lot, since logs never have perfectly consistent dimensions.
There's a lot of truth to this. I'm currently reading a book called "Norwegian Wood: Chopping, Stacking, and Drying Wood the Scandinavian Way" by Lars Mytting and he argues this point a lot. The density of wood can vary a lot within the same tree species due to the specific local conditions for each individual tree. He makes the claim that we should be less concerned with which species are more dense and focus more on the species that are easier to harvest and process (as long as the local conditions for such trees are conducive to producing denser wood). From his book: " Tables recording the density of the different types of wood are plentiful, but we should not be blinded by statistics. These are averages only, and the local variations for each type of tree can be considerable. Generally speaking, we can say that conifers that have grown slowly in poor soil will be better and heavier than those that have shop up quickly in rich, damp, low-lying soil. The latter will be lighter and less compact, as can be seen from the wider spacing between annual rings. The opposite is true for oak, hickory, ash, elm, and other so called ring-porous woods, which will become harder and denser when growing fast. Forestry researchers in Norway have recorded large variations in density of conifers, particularly in spruce and pine. The average density is 840 pounds and 970 pounds respectively, per cubic meter. However, researchers have also come across examples of both that weigh 1,320 pounds per cubic meter, which is the density typical of oak."
Thank you for sharing this! He says elm should be denser when it grows fast, however, so many I'm imagining things. But I get your broader point, that it is important to consider local conditions when evaluating the relative quality of firewood. I absolutely agree.
Hope you like it! I'm currently about half way through and I'm really enjoying it. It's interesting to hear what cultural significance they put on harvesting, drying, and burning wood. My kind of people.
After heating my house only with wood for 29 years, I am less concerned with wood heat content. Minimum heat content for me is American Elm (white) or better. Density varies for a given specie, moisture varies for every stick, the size of each stick varies, flue draft varies with atmospheric conditions, the shape of the burn pile varies, the level of ash changes... Drying time is more important for me than heat content, the quicker the better. Burning a blend of specie makes most wood generic. I struggle to heat with wood lighter than American Elm, like basswood. I heat with a wood furnace and so burning fast and hot enough to engage the circulating blower (1000 cfm) is my primary goal. I do not overnight coals since our bed is above the furnace and coals are too hot for sleeping.
I’m understanding that most of those tables were done by taking a sample and actually burning it and going through the scientific process of actually measuring the btu outputs. gotta have something for them brainiac college nerds to do for their science papers.
Could be. That's a tough way to go about it though. The bottom line is, if all your wood is very well seasoned, the heavier it is, the more heat it has. Pretty obvious I guess.
