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Firewood Cord & BTU Heating Estimator

Calculate gross and net thermodynamic heat yield from firewood by species, volume, and stove efficiency — plus equivalents in natural gas therms and grid electricity.

Fuel Stockpile Parameters

🔥 OFFLINE SURVIVAL DIAGNOSTIC: Burning low-density softwoods like White Pine produces nearly half the thermal output of an equal volume of Oak. To survive a winter entirely off-grid without freezing, you must mathematically double your timber storage volume if you are forced to burn softwoods.

Net Usable Heat Energy

55.4M BTU
Millions of BTUs yielded.

Grid Electricity Equivalent

16,221 kWh
1 Million Net BTUs = 293.07 kWh.

Natural Gas Equivalent

554 Therms
Volume equal to municipal gas utilites.
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What is Firewood Thermodynamics & Fuel Equivalence?

Firewood heat output is measured in millions of BTUs per cord. A cord is a standardized volume of 128 cubic feet (4ft × 4ft × 8ft). The usable energy you actually get is always less than the gross rating because no combustion device is perfectly efficient — the difference goes up the chimney as heat, moisture, and incomplete combustion byproducts.

Mathematical Foundation

Gross Heat Energy
Egross=Vcords×HspeciesE_{gross} = V_{cords} \times H_{species}
EgrossE_{gross}= Total gross thermal energy (Millions of BTUs)
VcordsV_{cords}= Volume of wood in cords (1 cord = 3.62 cubic meters)
HspeciesH_{species}= Species heat density (M BTU/cord): Hickory=26.8, Red Oak=24.6, Hard Maple=24.0, White Birch=20.0, Douglas Fir=17.1, White Pine=14.3, Cottonwood=13.5
Net Usable Heat Energy
Enet=Egross×η100E_{net} = E_{gross} \times \frac{\eta}{100}
EnetE_{net}= Net usable heat energy (Millions of BTUs)
η\eta= Stove combustion efficiency (%). A modern EPA-certified wood stove achieves 75–85%. An open fireplace may only be 15–30%.
Grid Electricity Equivalence
EkWh=Enet×293.07E_{kWh} = E_{net} \times 293.07
EkWhE_{kWh}= Equivalent energy in kilowatt-hours
293.07293.07= Thermodynamic constant: 1 Million BTU = 293.07 kWh
Natural Gas Equivalence
Etherms=Enet×10E_{therms} = E_{net} \times 10
EthermsE_{therms}= Equivalent natural gas therms
1010= Conversion constant: 1 Million BTU = 10 therms (1 therm = 100,000 BTU)

Laws & Principles

  • Density Determines Everything: Hardwoods have denser cell structures than softwoods. Burning equal volumes of Hickory vs. White Pine produces nearly twice the heat output. Volume alone is a useless metric — species always matters.
  • Moisture Content is Critical: All BTU ratings assume 'seasoned' (air-dried) wood at roughly 20% moisture content. Green (freshly cut) wood can have 50%+ moisture. Up to 25% of the gross BTU content is consumed simply boiling off that excess water before combustion can begin cleanly.
  • Open Fireplace Efficiency Warning: A traditional masonry fireplace is only 15–30% efficient and can actually depressurize the house in cold weather, pulling in cold outside air to replace the hot air going up the chimney — often resulting in a net heat loss.
  • Creosote & Safety: Burning wet wood or softwoods at low-throttle smoldering temperatures (under 250°C) produces high levels of creosote — an oily, flammable residue that deposited in chimneys is the leading cause of chimney fires.
  • Off-Grid Planning Rule: A well-insulated 1,500 sq ft home in a cold climate typically requires 3–6 cords of hardwood (Red Oak or better) to heat through a full winter using a high-efficiency wood stove.

Step-by-Step Example Walkthrough

" A remote cabin owner in Vermont wants to know how many cords of Red Oak they need to match 800 therms of propane heat for the winter. Their wood stove is rated at 75% efficiency. "

  1. 1. Convert propane/gas target: 800 therms = 80 Million BTUs of net heat needed.
  2. 2. Account for stove efficiency: Gross BTUs needed = 80 M BTU ÷ 0.75 = 106.7 M BTU gross.
  3. 3. Divide by species rating: Red Oak = 24.6 M BTU/cord. Cords needed = 106.7 ÷ 24.6 = 4.34 cords.
  4. 4. Add a 15% buffer for weather variability and moisture: 4.34 × 1.15 = 4.99 cords.
  5. 5. Round up to a full cord for safety margin: Stock 5 full cords of seasoned Red Oak.
Final Result: 5 cords of seasoned Red Oak, split and stacked with 12+ months of air-drying time, will reliably match the heat budget while providing a safety margin for extreme cold snaps.

Quick Answer: How many BTUs are in a cord of wood?

The heat energy in a full cord of firewood (128 cubic feet) is entirely dependent on the tree species and your stove's efficiency. A cord of premium seasoned Red Oak generates roughly 24.6 Million gross BTUs, while a cord of softwood like White Pine generates only 14.3 Million BTUs. Using the Firewood BTU Estimator above, you must map your exact wood species and multiply it by your wood stove's combustion efficiency (e.g., modern EPA stoves hit 75%, while open fireplaces only yield 15%). The calculator will automatically translate your net usable heat into direct electrical (kWh) and natural gas (Therms) equivalencies for off-grid winter planning.

Off-Grid Heating Catastrophes

The Softwood Exhaustion

A homesteader living in the Pacific Northwest calculates they need 80 Million BTUs to survive a brutal mountain winter. They cut and stack three cords of abundant Douglas Fir (17.1 M BTU/cord), assuming three cords is always enough. However, they are using an old, non-catalyzed steel stove running at only 50% efficiency. Their net yield is a disastrous 25.6 Million BTUs total. By early January, their woodshed is entirely empty. To survive the winter with that stove, the calculator proves they actually needed an astronomical 10 FULL cords of Douglas Fir.

The Open Fireplace Illusion

A suburban family attempts to heat their entire 2,000 sq ft home using their traditional masonry open fireplace during a severe blizzard power outage. They burn beautiful, dense Hickory (26.8 M BTU/cord). However, an open fireplace possesses a dismal 15% efficiency rating and aggressively drafts the home's pre-existing warm air straight up the chimney. They process an entire half-cord of premium hardwood in three days, yet the ambient temperature in the bedrooms plummets to 45°F. They needed an EPA-certified insert to trap and radiate the heat.

Firewood Species Gross BTU Benchmark Table

Tree Species (Seasoned) Gross Heat Yield (Millions of BTU / Cord) Coal Volatility & Coaling Properties
Shagbark Hickory / Ironwood26.8 to 27.7 M BTUExceptional (Burns hot all night)
White Oak / Red Oak / Beech24.0 to 26.0 M BTUExcellent
White Ash / Yellow Birch23.6 M BTUGood
Douglas Fir / Tamarack17.1 to 19.0 M BTUFair (Burns fast)
White Pine / Hemlock / Aspen13.5 to 14.5 M BTUPoor (Flash fire, pure ash)

Note: Burning "Green" (unseasoned) wood drops these BTU ratings by up to 25% due to the massive thermal energy wasted boiling the trapped water out of the log.

Pro Tips for Maximizing Timber Heat Yield

Do This

  • ✓Factor in thermal mass. Using a cast iron or soapstone wood stove drastically increases your effective efficiency. While the combustion might hit 75%, the thick stone mass absorbs the fierce heat and slowly radiates it into the cabin for 8 hours after the fire dies, preventing extreme temperature swings.
  • ✓Season hardwood for 2 years minimum. Dense species like Red Oak act like rigid sponges. Stacking them outdoors for only 6 months guarantees they will still possess over 30% moisture. Splitting the logs immediately and top-covering them in sunlight for 24 months is mandatory to hit the 24.6 M BTU rating target.

Avoid This

  • ✗Never burn soft pines at night. Avoid loading your stove with White Pine or Spruce right before bed. These low-density woods lack the cellular structure to create "coals". Instead, they flash-burn instantly, bringing the room to 90°F in forty minutes, before rapidly burning out, leaving you waking up to a freezing house at 3:00 AM.
  • ✗Don't confuse Face Cords with Full Cords. If you buy wood from an unreliable supplier, they often sell a "Face Cord" (a stack measuring 4x8 feet, but only 16 inches deep, totaling 42 cubic feet). A true mathematical "Full Cord" is 128 cubic feet. Buying Face Cords mathematically guarantees you will run out of fuel.

Frequently Asked Questions

What is the definition of a full cord of firewood?

A full cord is a legally standardized volumetric measurement of stacked timber measuring exactly 128 cubic feet. Typically, this is organized into a tight rank of logs that is 4 feet high, 8 feet long, and 4 feet deep. Due to the air gaps between the logs, a 128 cubic foot cord realistically contains about 85 to 90 cubic feet of actual solid wood.

Why does firewood efficiency drop so heavily with moisture?

Water cannot combust. If a piece of Oak contains 40% microscopic water by volume, the fire must first physically boil and vaporize every single ounce of that water into steam before the wood fiber itself can catch fire. You are utilizing a massive portion of the wood's own stored BTUs simply to act as a tea kettle, severely cannibalizing the net heat radiating into your room.

How many therms of natural gas equal one cord of Red Oak?

A full cord of seasoned Red Oak contains roughly 24.6 Million gross BTUs. Because 1 therm of natural gas perfectly equates to 100,000 BTUs, generating 24.6 Million BTUs of raw heat would require burning approximately 246 therms of natural gas (assuming both setups were operating at a fictional 100% efficiency).

Is hardwood actually better than softwood?

Yes, purely due to biological density. A pound of White Pine produces the exact same amount of thermal energy as a pound of Hickory. The problem is volume. Because Hickory is intensely microscopically dense, a single 16-inch log of Hickory weighs significantly more than the same size Pine log. Therefore, it holds drastically more BTUs per cubic foot, requiring you to chop, stack, and haul far fewer cords.

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