BTU Values of Wood Species for an Outdoor Wood Boiler

Every outdoor wood boiler is rated in BTU (British Thermal Unit) output — the energy needed to raise one pound of water by one degree Fahrenheit. A full cord of firewood is a stack measuring 4' high, 4' wide, and 8' long (128 cubic feet), typically cut and split to 16" lengths. A "face cord" is only 1/3 of a full cord — worth knowing before you buy.

Species is the biggest factor in a cord's BTU value. According to the U.S. Department of Energy, a cord of air-dried hardwood produces 18–32 million BTUs, while softwood produces 12–18 million BTUs. For comparison, propane delivers about 91,000 BTU per gallon — making a cord of high-quality hardwood roughly equal to 300 gallons of propane. Moisture content matters just as much: unseasoned wood burns cooler, smokier, and leaves more creosote. Season wood 6–12 months down to about 20% moisture for the best results.

BTU Per Cord by Species (seasoned, sorted high to low)

Softer woods near the bottom of the chart tend to burn faster and leave more creosote than the hardwoods at the top — worth factoring in alongside price and availability when you're stocking up.