For a home heated primarily with wood, the key is how many BTUs a stack delivers after moisture and efficiency losses. A cord is a volume measure, so two cords can differ by more than double in usable heat. The math below converts stack volume into delivered heat and estimates winter cord requirements.
Cord Volume and Solid Wood Fraction
A full cord is 4 feet high, 4 feet wide, and 8 feet long (128 cubic feet). The term does not describe the amount of solid wood inside. A tightly stacked cord typically contains 80 to 85 cubic feet of solid wood; a loose pile can drop to roughly 70 cubic feet. Species density converts this volume to weight. Dense white oak at 20 percent moisture runs about 40 pounds per cubic foot, so a cord with 80 cubic feet of solid wood weighs around 3,200 pounds. Softwood like ponderosa pine may weigh only 2,200 to 2,500 pounds for the same stack volume, directly reducing available energy.
Moisture Content and Heating Value Penalty
Moisture is the largest factor in heating value. Bone-dry wood contains roughly 8,500 BTU per pound, but water must evaporate before combustion. Every 1 percent increase in moisture reduces effective heating value by about 1 percent: wood at 20 percent moisture delivers roughly 7,100 BTU per pound, while at 50 percent it falls to around 4,250 BTU per pound. Green white oak at 3,500 pounds might contain 1,200 pounds of water, consuming energy before useful heat reaches the room. Seasoned wood should test near 20 percent moisture. Split pieces dry faster than unsplit rounds; 6 to 12 months under cover is typical, while dense oaks require a year or more. Ground-stacked or uncovered wood absorbs rainfall and rehydrates.
Species BTU Tables and Density Spread
Extension service tables for air-dried wood at 20 percent moisture report million BTU per cord. Osage orange tops the list at 30.7, followed by shagbark hickory at 28.0, black locust at 27.9, and white oak at 25.7. Sugar maple and red oak both come in at 24.0, while white ash is 23.6. The lower end includes black cherry at 19.9, quaking aspen at 18.2, ponderosa pine at 16.2, eastern white pine at 15.9, and cottonwood at 15.8. The spread from cottonwood to osage orange is roughly a factor of two: a home needing four cords of osage orange would need nearly eight cords of cottonwood for the same heat. This spread is due to density, not chemical composition; all species have similar BTU per pound when bone dry.
Face Cords and Piece Length Fractions
A face cord (rick) is 4 feet high and 8 feet long but only as deep as the pieces are cut. With 16-inch pieces, the volume is 42.7 cubic feet, or one third of a full cord; three face cords make one full cord. With 24-inch pieces, it is 64 cubic feet, so two face cords equal one full cord. Without stating piece length, face cord is ambiguous; buyers should always confirm length. Comparing cost per million BTU is more useful than cost per cord. Dividing cord price by the million BTU figure gives a direct comparison across species and moisture.
Appliance Efficiency and Delivered Heat
Efficiency varies widely. A modern EPA-certified stove may reach 70 to 80 percent efficiency, while an open fireplace operates at 10 to 15 percent, wasting most heat. For a house needing 60 million BTU, a red oak cord at 24 million BTU through a 70 percent efficient stove yields 16.8 million BTU of usable heat, requiring about 3.6 cords. Through a 12 percent efficient fireplace, each cord delivers only 2.88 million BTU, requiring nearly 21 cords. Actual performance depends on loading, refueling, and damper use, differing from steady-state ratings.
Seasonal Demand and Supply Sizing
Seasonal heat demand depends on climate, house size, insulation, windows, and setpoint. Estimate winter heat loss in million BTU and divide by delivered heat per cord. A cold climate home heated mainly by a certified stove typically needs 3 to 5 full cords; a smaller, well-insulated house might need 2 to 3, while a larger or poorly insulated home may require 6 to 8. Use actual species and moisture level, not averages. Buying wood in spring or summer allows time to dry; fall-delivered wood may not season in time. Check moisture with a meter, targeting 20 percent.
Practical Checklist for Cord Math
Measure stack dimensions to confirm volume. A stack 4 feet high, 8 feet long, and 20 inches deep is 53.3 cubic feet, or about 42 percent of a cord. Always ask for piece length when buying by the face cord. Estimate solid wood fraction: assume 80 to 85 cubic feet per full cord for tightly stacked hardwood, adjusting downward for loose stacks. Multiply by species density to get cord weight. Apply moisture penalty: use 7,100 BTU per pound for 20 percent moisture wood, reducing by 1 percent per 1 percent moisture increase. Calculate delivered heat per cord by multiplying total BTU by appliance efficiency (e.g., 0.70 for stove, 0.12 to 0.15 for fireplace). Divide seasonal demand by delivered heat per cord to find required cords.