Purchasing a wood heater is a big commitment. Whether you’re getting a wood stove or a wood burning furnace, there are so many options available. It can be hard to sift through all the models, features, and prices of these machines to find the perfect one for your home.
But there’s a question that’s even more fundamental, no matter which kind of wood heating appliance you’re planning to purchase: how much wood will I need to heat my home, anyway?
This is perhaps the most important question to consider when buying a wood furnace or wood burning stove. It’s impossible to answer with absolute certainty due to a wide range of factors like your home’s insulation, the location of the appliance in your home, how the appliance is installed, the climate you live in, and plenty more.
But we can certainly point you in the right direction.
In this guide, we’re going to outline a general formula you can use to estimate how much wood your heating appliance will use. We’ll explain which numbers to use, where to find them, and how to convert them into the figures you need.
By the time you’re finished here, you’ll be armed with the knowledge to find out roughly how much wood your stove or furnace will require to get you through the winter.
How to Calculate the Amount of Firewood You Need
Before we get started, we should make sure you’re familiar with two units known to veteran wood burners everywhere:
- BTUs
- Cords
Used often in wood heating, British thermal units (or BTUs) refer to the quantity of heat required to raise the temperature of a pound of water by 1 degree Fahrenheit. In simpler terms, 1 BTU is equal to about the amount of energy released by burning a match.
We’ve covered cords and face cords before, but generally speaking, a cord of firewood:
- Has a volume of 128 cubic feet
- Measures about 4 feet tall x 8 feet wide x 4 feet deep
- Weighs about 3,000 to 5,000 pounds
- Contains about 600 to 800 logs
You can expect to burn anywhere from 2 to 10 cords of firewood in your wood heating appliance over the course of the year. Obviously that’s a pretty wide range — anywhere from about 1,500 to 7,500 logs. We need to start narrowing that amount down, and we can do so with a formula:
To figure out how many cords of wood your heating appliance requires, you’ll need to divide the total number of BTUs needed to heat your home over the course of a year by the number of BTUs in a cord of your chosen wood species multiplied by the efficiency of your heating appliance.
If that sounds complicated, let’s simplify it by extracting each number and plugging them into the formula.
BTUs Needed to Heat Your Home
If you’ve lived in your home for at least a year and have been using a natural gas heater, you should be able to acquire this number from your natural gas supplier or your billing history. Let’s take a look at some real-world numbers from the annual heating bill of a home in St. Louis, Missouri:
This home used 506 CCF of natural gas over the course of the year in 2023. CCF is equal to 100 cubic feet, and it’s a common measurement used by utility companies to quantify natural gas use.
There are about 104,000 BTUs in 1 CCF of natural gas. So, in 2023, this home required 52,624,000 BTUs (104,000 x 506) to heat it.
BTUs per Cord of Firewood
Not all firewood is created equally. By virtue of its tree species, some firewood puts off more heat than others. You should easily be able to figure out, on average, how many BTUs of energy are in a cord of a specific species of firewood.
Let’s use maple firewood as an example. Several factors determine how many BTUs there are in a cord of maple wood, including its moisture content, its size, and how long it’s been allowed to cure.
But, on average, a cord of maple firewood emits 25,500,000 BTUs of heat.
Efficiency of Your Wood Heating Appliance
Knowing how many BTUs are in a cord of firewood gets us close to knowing how many cords we’ll need, but we’re not quite all the way there. We also need to consider the efficiency of a wood stove or wood furnace.
In a perfect world, if you were to burn 25,500,000 BTUs worth of wood in your furnace or stove, you’d receive 25,500,000 BTUs of heat energy in return. Unfortunately, the real world doesn’t work that way. In reality, the efficiency of a wood heater isn’t perfect, and some of the heat is lost through the chimney flue or otherwise radiated away.
Take our Fire Chief FC1000E wood burning furnace, for example. It’s an EPA-approved wood burning furnace, meaning its efficiency is very high for its class. Even still, it only has an efficiency rating of 79%.
That means if you were to burn 25,500,000 BTUs worth of wood in it, you’d only receive 20,145,000 BTUs (25,500,000 x 0.79) of heat energy in return.
While losing over 5,000,000 BTUs of heat energy seems like a lot, these inefficiencies are simply a reality in heating and cooling, no matter what appliance you’re using. Even some natural gas heaters have an efficiency rating as low as 78%.
This efficiency metric is essential for our word cord formula, and it’s usually readily available from the manufacturer of your wood heating appliance.
Running the Numbers
So, in our example, we’ve discovered that:
- Our home requires 52,624,000 BTUs to heat it
- One cord of our chosen firewood contains 25,500,000 BTUs of energy
- Our wood heating appliance has an efficiency of 79%
- That means we’ll get 20,145,000 BTUs out of a cord of our chosen firewood
All that’s left to do complete the equation:
According to all of the variables, a Fire Chief FC1000E requires about 2.61 cords of maple firewood to get us through the winter.
To make these numbers work for you, you’ll have to swap out the BTUs needed to heat your home over a year, the BTUs in a cord of your chosen firewood, and the efficiency percentage of your wood heater.
Is It Okay to Store Even More Firewood?
And that’s how to figure out how much firewood you need to operate a wood furnace or wood stove through the heating season. Keep in mind that these numbers are rough approximations. There are several other heating and cooling factors to take into consideration.
With that in mind, you may be wondering: is it a good idea to err on the side of more firewood?
The answer is yes! You can never have enough firewood on hand, especially because the longer firewood sits and cures, the better it will burn.
Keep in mind, though, that some species of firewood are better for wood burning than others. Our guide to the best firewood outlines how much energy you should expect to get out of several common species to help you find the right firewood to use in your appliance.
