Many companies, such as The Chimney Sweep and World Forest Industries, publish the average BTUs per cord of firewood for various hardwood and softwood species. In general, hardwoods, such as oak or eucalyptus, have higher BTU ratings than softwoods, such as pine or fir.
The Chimney Sweep's online chart ranks varieties of firewood on BTUs per cord and lists them in alphabetical order. The website reports that Osage orange, a variety of hedge, is the highest-energy firewood and contains 30 million BTUs per cord. Conversely, balsa, a very light, low-density wood, contains 5.8 million BTUs per cord.
BTU output rankings on the World Forest Industries site vary significantly. It gives the top position to live oak, with an output of 36.6 million BTUs per cord. The lowest BTU firewood on its chart is eastern white cedar, which is rated at 12.2 million BTUs per cord. The company attributes this inconsistency to variables in laboratory conditions.
The British Thermal Unit is the standard measurement for fuel sources. A BTU is defined as the amount of heat energy necessary to raise 1 pound of water 1 degree Fahrenheit. Most firewood ratings are given per cord of firewood, which is a volume of firewood measuring 4 feet wide, 4 feet high and 8 feet long. After accounting for space between logs, the actual volume of cut wood in a cord is approximately 85 cubic feet. Green wood contains the same amount of energy as seasoned wood, but emits less heat, because much of the energy is consumed as the moisture in the wood evaporates.