Wood has served as a primary heating fuel for centuries, and its energy value is quantified to understand its efficiency and cost-effectiveness. The amount of heat energy contained within a given volume of wood is not fixed, but instead depends on multiple physical properties. Quantifying the potential energy of a standard stack of firewood requires understanding the specific units used for both heat measurement and volume, providing a baseline for homeowners to compare wood to other modern heating fuels.
Defining Heat Measurement and Volume
The standard measure for thermal energy in the United States is the British Thermal Unit (BTU). This unit represents the energy required to raise the temperature of one pound of water by one degree Fahrenheit. When discussing fuel, the BTU rating indicates the total heat released during complete combustion, making it the standard metric for comparing different energy sources.
The volume of firewood is standardized by the cord, a measurement used throughout the industry. A full cord is defined as a stack of wood measuring four feet high, four feet wide, and eight feet long, totaling 128 cubic feet. A cord is a measure of stacked volume, which includes air space between the pieces, not solid wood volume. Since the BTU value is determined by the mass of the wood fiber, this air space is a primary reason the heat output of a cord is variable.
The Average Heat Content of a Cord
The total heat energy contained in a cord of wood can vary significantly, ranging from about 12 million to over 32 million BTUs. This wide spectrum is primarily divided between hardwoods and softwoods due to their different densities. Hardwoods, such as oak and maple, are denser and contain more wood fiber per cord, which translates to a higher potential heat output.
A cord of air-dried hardwood generally produces between 18 million and 32 million BTUs, averaging around 21.3 million BTUs. Conversely, softwoods, like pine and spruce, are less dense and yield less heat energy per unit of volume. A cord of softwood typically provides between 12 million and 18 million BTUs, with an average around 15.4 million BTUs. These figures represent the potential heat content, assuming the wood has been properly prepared for burning.
Factors That Determine Actual Heat Output
The two most significant factors dictating a cord’s actual heat output are the wood species’ density and the wood’s moisture content. Density refers to the mass of the wood fiber packed into a given volume, determining the amount of carbon and hydrogen compounds available for combustion. High-density hardwoods like hickory, oak, and beech contain substantially more total mass and, therefore, a greater number of BTUs than low-density softwoods like cedar, poplar, or pine.
Moisture content is an equally important factor because water must be evaporated before the wood can burn efficiently. Freshly cut, or “green,” wood can have a moisture content of 50% or higher. The energy required to boil off this water is diverted from heating the home, which significantly reduces the net heat output and leads to incomplete combustion and increased smoke. For optimal heating efficiency and maximum BTU output, wood should be properly seasoned to achieve a moisture content of 20% or less.
Comparing Wood Fuel to Other Energy Sources
The average BTU content of a cord of seasoned, dense hardwood allows for a direct comparison with the energy content of common residential heating fuels. A standard cord of high-quality hardwood contains the energy equivalent of a substantial volume of fossil fuels, which can be measured against liquid and gaseous fuels to determine cost-effectiveness.
The heat energy in one cord of seasoned dense hardwood is roughly equivalent to the energy found in about 155 gallons of heating oil or approximately 225 therms of natural gas. The same cord is also comparable to about 215 gallons of propane. Comparing wood to these alternative fuels is often done by calculating the cost per million BTUs, which standardizes the energy output and allows consumers to make informed economic decisions regarding their heating source.