Firewood Calculator

Estimating a winter's firewood means converting a heat demand into a volume of wood. The demand side depends on floor area, envelope quality, the length of the heating season and the share of heating the stove actually carries. The supply side depends on the fuel and the appliance. A cord is a stacked volume of 128 cubic feet, 4 feet by 4 feet by 8 feet, of which only about 70 to 90 cubic feet is solid wood once the air gaps between round and split pieces are subtracted. Heat content tracks density almost linearly: white oak and hickory deliver roughly 27 to 29 million BTU per cord, sugar maple and ash around 23 to 24 million, birch near 20 million, and pine only 15 to 17 million. The appliance decides how much of that reaches the room. An open masonry fireplace converts only 10 to 20 percent of the fuel's energy into usable heat and can pull more warm household air up the flue than it contributes. An EPA-certified wood stove runs 60 to 80 percent efficient, so the same cord does three to five times more work. Moisture is the other lever. Green wood carries 45 to 60 percent moisture by dry weight, and driving that water off consumes heat that never reaches the room, so wood must be air-dried to below 20 percent, typically 6 to 12 months for softer species and 18 to 24 for oak. Burning wet wood also condenses creosote in the flue, the cause of most chimney fires.

The cord is one of the oldest surviving customary units still in commercial use. The name refers to the cord or string once used to bind and gauge a stack, and the 4 by 4 by 8 foot definition was established in English practice and carried into American law. Because stacked volume can be manipulated by loose piling and because dealers commonly sell partial stacks under regional names such as rick or face cord, firewood is a regulated commodity: the model method-of-sale regulation in NIST Handbook 130, adopted by most states, requires firewood to be sold by the cord or fraction of a cord with stated dimensions rather than by truckload or by an unqualified nickname. The efficiency side of the calculation is a story of appliance design. Benjamin Franklin's 1741 iron fireplace attempted to capture heat that was otherwise lost up the chimney, and in the 1790s Benjamin Thompson, Count Rumford, redesigned the fireplace with a shallow firebox and angled sides to radiate more heat into the room and a narrowed throat to reduce draft losses. The decisive change was regulatory: the Environmental Protection Agency issued its first new source performance standard for residential wood heaters in 1988, which forced catalytic combustors and secondary air designs into the market, and a further tightening phased in by 2020 limits certified stoves to about 2.0 grams of particulate per hour, with efficiency gains as a side effect.