About Firewood Cord Calculator (Cords, Face Cords & BTU)
The firewood cord calculator answers "how much wood is that stack, really?" Measure the stack's length and height in feet and the piece (log) length in inches, and the tool computes the volume and divides by 128 ft³ — the legal definition of a cord under NIST Handbook 130's Uniform Regulation for the Method of Sale of Commodities: "the amount of wood that is contained in a space of 128 cubic feet, when the wood is ranked and well stowed." Most US states adopt this regulation, which is why sellers must quote firewood in cords or fractions of a cord, not "truckloads," "racks," or "piles."
The tool also converts the stack to face cords — a 4 × 8 ft face, one piece deep — and shows what fraction of a full cord one face cord at your piece length actually is, because that fraction is where buyers get burned: a face cord of 16″ wood is exactly 1/3 of a cord, but a face cord of 12″ wood is only 1/4. Pick the species to see the heat content in million BTU (from Utah State University Forestry Extension's heating table, dry-wood basis), and enter the asking price per cord to compare wood deals on the only fair axis: dollars per million BTU.
How It Works
- Measure the stacked pile: length along the row and average height, in feet. The wood should be ranked and well stowed — pieces parallel, touching, compactly stacked — the same condition the legal cord assumes. A loosely thrown pile holds noticeably less wood in the same space.
- Enter the piece length in inches (12–24″ covers common stove and fireplace wood; 16″ is the most common sale length). The stack is one row deep, so the piece length is the stack depth: volume = length × height × piece length ÷ 12.
- Read the cords headline: volume ÷ 128. The face-cord lines show how many 4 × 8 ft single-row faces your stack equals at its piece length, and what fraction of a full cord each such face cord represents — the number to check against a seller's "cord" claim.
- Pick the species to get total million BTU (cords × the species' MBTU per cord, dry-wood basis). Denser wood packs more heat into the same 128 ft³ — white oak carries nearly twice the heat of cottonwood per cord.
- Optionally enter the price per cord: the tool reports the total cost of the stack and the price per million BTU, the honest way to compare a cheap light-wood cord against a pricier oak cord — or firewood against any other fuel.
Worked Example
A seller delivers a stack 16 ft long and 4 ft tall of 24″ logs. Volume = 16 × 4 × (24 ÷ 12) = 128 ft³ = exactly 1.000 cord. At 24″ pieces, one face cord is 32 × 2 = 64 ft³, so the stack equals 2.0 face cords, and each face cord is exactly 1/2 of a full cord. As white oak (29.1 MBTU per dry cord) the stack holds 29.1 million BTU; at $300 per cord it costs $300 total, or $300 ÷ 29.1 ≈ $10.31 per million BTU.
Formulas
- Stack volume
V = L × H × (P / 12)- Full cords
cords = V / 128- Face cord volume (depends on piece length!)
V_face = 32 × (P / 12)- Face cords and fraction of a full cord
faceCords = V / V_face; fraction = V_face / 128- Heat content
MBTU = cords × MBTU_species- Cost
total = cords × price; $/MBTU = price / MBTU_species
Standards & References
- NIST Handbook 130, Uniform Regulation for the Method of Sale of Commodities, §2.4 Fireplace and Stove Wood — cord = "the amount of wood that is contained in a space of 128 cubic feet, when the wood is ranked and well stowed"; adopted by most US state weights-and-measures programs
- Utah State University Forestry Extension, "Wood Heating" — million BTUs per cord of dry wood by species (heat-content table)
- Dry (seasoned) wood basis: green wood delivers less usable heat because combustion must first boil off the water
Frequently Asked Questions
How much is a cord of wood?
A cord is 128 cubic feet of wood ranked and well stowed — pieces parallel, touching, and compactly stacked — per NIST Handbook 130's Uniform Regulation for the Method of Sale of Commodities, adopted by most US states. The classic picture is a stack 4 ft tall × 8 ft long × 4 ft deep, but any L × H × depth multiplying to 128 ft³ counts: the worked example's 16 ft × 4 ft stack of 24″ logs is exactly one cord.
What is a face cord, and why is it a buyer-beware unit?
A face cord is a 4 × 8 ft face of stacked wood one piece deep — so its volume depends entirely on the piece length. At 16″ pieces it is 42.7 ft³, exactly 1/3 of a full cord; at 12″ pieces only 1/4. The classic firewood mistake is paying "half a cord" money for a face cord of short wood. Always convert to full cords (or cubic feet) before comparing prices — that conversion is exactly what this calculator does.
Why does the species matter if a cord is always 128 ft³?
Because a cord is a volume, not a weight, and denser wood packs more combustible mass into the same 128 ft³. Per USU Forestry Extension's table, a dry cord of white oak holds about 29.1 million BTU while cottonwood holds 15.8 — nearly a 2:1 spread. All species release similar heat per pound of dry wood; the cord-to-cord difference is almost entirely density.
Do these BTU numbers apply to green (unseasoned) wood?
No — the table is for dry wood. Green wood can be one-third water or more, and that water must be boiled off in the firebox before the wood burns, stealing heat and promoting creosote. Season firewood split and top-covered for 6–12 months (dense oaks often need a year or more) until moisture drops to roughly 20% before counting on the full heating value.
Is $300 for a cord of oak better than $200 for a cord of cottonwood?
Compare dollars per million BTU, not per cord: $300 ÷ 29.1 ≈ $10.31/MBTU for white oak versus $200 ÷ 15.8 ≈ $12.66/MBTU for cottonwood — the "cheaper" cord costs about 23% more per unit of heat. Enter the asking price in the tool to get this number directly, and remember the light wood also means more loading, more storage, and more ash for the same warmth.
How does firewood compare with my furnace or heat pump?
This tool prices the wood itself in $/MBTU; a full fuel comparison also needs the appliance efficiency (a modern EPA wood stove converts roughly 70–75% of that heat into the room) and how much heat your house actually needs. Estimate the seasonal demand with the degree-days and heat loss/gain calculators, and size equipment with the HVAC load calculator — then multiply your $/MBTU by 1/efficiency for the delivered-heat cost.