Firewood BTU Simplified
You can generally tell how much heat firewood can produce by its density. The less dense a wood is, the less wood there is to burn, the less total heat it can produce. The more dense it is, the more wood it has to burn, the more total heat it can produce. If a piece of wood is heavier than another similar size piece of wood, the heavier one will have roughly that much more heat energy. Congratulations, you’re now about 90% of the way to being a firewood BTU expert.
People Are Confused Over Firewood BTU
A lot of people are really confused over firewood BTU. And for good reason. There are a lot of conflicting firewood BTU charts out there. Here are some below compiled from various sources. I think some of the figures are questionable, but I think these can give you a general idea.
Mostly knowing which wood is lighter and which is heavier per volume will tell us most of what we need to know. If you scroll below the charts we’ll cover some of the exceptions to that.
Western Softwoods Figures from California Energy Commission. Rating Based on 90 cubic feet of solid wood per 128 cubic foot cord
Eastern Hardwoods compiled from various sources. Consistency is…. um….Inconsistent. They may be in the right order but I think the BTU is low when compared to the Western charts. I have intentions to look into this and fix it. But you know what they say about intentions.
Eastern Softwoods
Compiled from various sources. Consistency between charts is, let’s just say it’s interesting.
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What Is a BTU Anyway?
BTU stands for British Thermal Unit. It is a unit for measuring heat energy based on the amount of body heat created by one British person. One BTU is the amount of body heat an average British person produces per minute, exactly one hour after drinking tea.
It’s also claimed to be the amount of heat needed to increase one pound of liquid water by 1 degree Fahrenheit (F). Most people would claim this last one is the more official one and the one above is complete bunk. But if it’s a British unit, why is it based on pounds and Fahrenheit instead of the metric system? It seems a little sus to me.
Softwood is Higher In BTU Than Hardwood
Contrary to common belief, softwoods typically have higher BTU than hardwoods per pound. That’s because softwoods commonly contain resins, which have more BTU than wood fiber. If you have a piece of hardwood and a piece of softwood of equal size and weight, the softwood will commonly have a little more total heat potential.
But since more dense hardwoods typically have so much more density than the softwood, they far surpass that small difference. Because of that, it still holds true that dense hardwoods have higher BTU per volume than less dense softwoods. Softwoods typically have slightly more BTU per weight than hardwoods, dense hardwoods typically have more BTU per volume than less dense wood.
When it comes to volume, BTU is not based so much on hardwood vs softwood. Density matters more than hardwood vs softwood
But Not Always
It seems like with everything, there are always exceptions. I think there are even exceptions to exceptions. So it goes with firewood and BTU.
Sometimes tree bark can throw a monkey wrench into things. Some dense hardwood species can have bark that is much less dense than the wood. If you get a cord of wood that has a lot of less dense bark still attached, that can reduce the number of BTU per cord. It can also work the other way. Some bark can be denser than the wood. That could increase BTU per cord. It all depends on the species.
Some types of wood produce more ash than others. If a species produces more ash, we have to subtract a few BTU for that. But it’s not enough that I would lose sleep over it or bother trying to figure out.
I think looking at the density of wood covers most of what we need to know about firewood BTU.