The cord is firewood’s legal measure, defined by NIST Handbook 130’s Uniform Regulation for the Method of Sale of Commodities as “the amount of wood that is contained in a space of 128 cubic feet, when the wood is ranked and well stowed” — pieces parallel, touching, compactly stacked. Most US states adopt the regulation, which is why sellers must quote in cords or fractions of a cord rather than “truckloads,” “racks,” or “piles.” Measuring a stack is arithmetic: length × height × piece length, then divide by 128.
The trap in firewood buying is the face cord — a 4 × 8 ft face stacked one piece deep — because its share of a real cord depends entirely on piece length. A face cord of 16-inch wood is exactly one third of a cord; cut the pieces to 12 inches and the same-looking face holds only a quarter. Checking a seller’s “cord” claim means converting their stack through its piece length, not its face.
Volume also is not heat: denser species pack more energy into the same 128 ft³, with white oak carrying nearly twice the heat of cottonwood per cord (dry-wood basis, per the Utah State Forestry Extension heating table). The fair comparison between wood deals — or between wood and any other fuel — is dollars per million BTU, not dollars per stack.