Firewood BTU & Cord Calculator

Firewood planning comes down to two things: how much wood do I actually have in that pile, and how much wood do I need to get through the winter. The first is a volume problem — a "cord" only means something if you know what the seller means by it. The second is an energy problem, and it depends on species, moisture, and how efficiently your appliance turns wood into heat in the room.

This tool does both. Measure a stack to see what fraction of a full cord it really is, then compare the heat it holds against your seasonal demand. Everything below explains where the numbers come from and where they stop being trustworthy.

A full cord is a defined volume: 128 cubic feet of stacked wood, classically 4 ft high × 8 ft long × 4 ft deep. That definition is what makes price comparison possible, and it is the only unit this calculator uses.

Almost everything else you will be offered is a fraction of that with a friendly name:

This is the practical use for the volume half of the calculator: measure what shows up in your yard, convert to full cords, and divide the price by that. Two sellers quoting "$150 a cord" can differ by a factor of three in delivered wood.

Stack length and height — the face of the stack you can see. Measure the average height, not the tallest point, and measure a stack that has actually been stacked; a thrown pile cannot be measured this way.

Piece length — the average cut length of the splits, which is the depth of the stack. This is the input that separates a full cord from a face cord.

Species — sets the heat value per cord. Where a load is mixed, either use the dominant species or run the calculation twice and split the volume.

Appliance efficiency — the fraction of the wood's energy that ends up as usable heat rather than going up the flue. Use the efficiency figure published for your specific stove, insert, furnace, or boiler. An open masonry fireplace is dramatically lower than a modern stove; an outdoor boiler with long buried lines loses heat the stove rating never accounts for.

Season heat demand — total heat your home needs across the heating season, in million BTU. The most defensible way to get this is from your own past fuel bills: last winter's actual propane, oil, gas, or electric consumption converted to MMBTU, adjusted for the efficiency of the appliance that burned it. Estimating it from square footage alone is guesswork.

Species values are published averages for well-seasoned wood near 20% moisture content, and they are already net of the air space in a normally stacked cord. They are averages across growing conditions and stacking tightness, not measurements of your wood.

A load arrives and you stack it neatly against the woodshed: 16 ft long, 4 ft high, cut to 16 in. Species is red oak. Your stove is rated near 70% efficiency, and last winter's propane bills worked out to about 60 MMBTU of delivered heat.

• Stacked volume: 16 × 4 × (16 ÷ 12) = 85.3 cu ft
• Full cords: 85.3 ÷ 128 = 0.67 cords — two-thirds of a cord, despite being a big-looking wall of wood
• Gross heat: 0.67 × 24.0 = 16.0 MMBTU
• Usable heat: 16.0 × 0.70 = 11.2 MMBTU
• Season requirement: 60 ÷ (24.0 × 0.70) = 3.57 cords of red oak

So this delivery covers under 19% of the winter. That is the number worth knowing in September rather than January. And if the seller called that stack "a cord," you paid for 1.0 and received 0.67.

Change one thing — burn cottonwood at 15.8 MMBTU instead of red oak — and the same 60 MMBTU season needs 5.42 cords. That is nearly two extra cords to cut, split, haul, stack, and handle for identical heat in the house. Species choice is a labor decision as much as a heat decision.

Published averages for seasoned wood near 20% moisture. Actual values vary with growing conditions, density within a species, and how tightly the cord is stacked. Softwoods are not "bad" — properly seasoned, they burn cleanly and suit shoulder seasons; they simply give less heat per unit of volume handled.

Species sets the theoretical ceiling. Moisture content determines how much of that ceiling reaches your house, and it swamps species differences in practice — badly seasoned oak loses to well-seasoned pine.

• Water has to be boiled off before wood burns. That energy comes out of the fire, not from somewhere else. Every pound of water in the split is heat that never reaches the room.
• Freshly cut wood can be half water by weight. The published BTU-per-cord figures assume roughly 20% moisture. Burning green wood does not just reduce output slightly — it changes how the fire behaves.
• Wet wood burns cool and dirty. Cooler flue temperatures let unburned volatiles condense in the chimney as creosote, which is the primary driver of chimney fires. This is a safety issue, not just an efficiency one.
• Split first, then season. Bark is designed to keep water in. Wood dries from the split faces, so rounds sitting whole for a year are often still wet inside.
• Stack for airflow, cover only the top. Off the ground, single rows if you can, ends exposed to sun and prevailing wind. Wrapping a stack in a tarp traps moisture and can be worse than no cover.
• Oak needs the longest. Many species are ready in a season; oak commonly wants considerably longer. Buying a year ahead is the single highest-return firewood habit.
• Measure instead of guessing. A moisture meter on a freshly split face — not the weathered outside — tells you the truth. Under 20% is the practical target.

This calculator has no moisture input, which is deliberate: it reports the seasoned-wood figure. If your wood is not seasoned, the honest move is to season it, not to apply a correction factor and burn it anyway.

Use the calculator's output to answer the decisions that actually come up:

• Comparing two sellers. Measure or confirm the stacked dimensions of each, convert both to full cords, then compare price per full cord. Never compare price per "load."
• Is cheap softwood worth it? Divide each option's price per full cord by its MMBTU per cord to get cost per million BTU. Softwood often wins on that basis and still loses on handling: more cords cut, split, stacked, carried, and more reloads through the night.
• Mixed woodlot planning. If your own woods give you a mix, save the dense species for deep winter and burn the lighter species in the shoulder months when you want a shorter, cooler fire.
• How much to cut this year. Take the season requirement, then add a genuine buffer — a cold winter, a wet season that keeps you out of the woods, or a chainsaw down for two weeks all eat reserve. Cutting for next year while burning this year's is the goal.
• Is the stove big enough? If the season requirement is high and you are reloading a small firebox constantly with light wood, denser species and a longer burn cycle solve more than more wood does.

Theoretical BTU in a cord and heat delivered into your rooms are different quantities, separated by several losses this calculator cannot see:

• Moisture content above the assumed 20%, which reduces delivered heat directly.
• Real-world operating efficiency, which is usually below the appliance's rated laboratory figure and depends on how you run the air controls.
• Distribution: a single stove heats the room it is in far better than the far end of the house. Unheated zones still consume from your demand figure if you make them up with another fuel.
• Flue and jacket losses, and for outdoor boilers, losses in buried supply lines.
• Your actual weather, thermostat habits, and how much of the season you supplement with another heat source.

The tool deliberately reports no dollar figure. A cost per million BTU is only meaningful with your own delivered wood price, your own appliance efficiency, and your own alternative fuel price — three numbers only you have. Compute cost per MMBTU as price per full cord ÷ (MMBTU per cord × efficiency) and compare it against the same figure for your alternative fuel.

• Species values are published averages, not measurements of your wood.
• No moisture input — results assume properly seasoned wood near 20% moisture.
• Volume math assumes a reasonably stacked, uniform stack. Crooked, knotty, or loosely thrown wood contains less wood per cubic foot.
• The season requirement assumes wood covers your entire heat demand at a constant efficiency.
• No pricing, labor, equipment, or emissions modeling.
• Nothing here replaces your appliance manual, clearance requirements, or a chimney professional's inspection.

• Paying full-cord prices for a face cord because the term was never defined at the point of sale.
• Measuring a thrown pile instead of a stacked one, which overstates volume substantially.
• Entering the appliance's peak efficiency rather than a realistic operating figure.
• Guessing seasonal demand from square footage instead of deriving it from last year's fuel use.
• Buying oak in autumn for that same winter.
• Tarping a whole stack to the ground and sealing moisture in.
• Assuming a hot-burning species compensates for wet wood. It does not — it just wastes better wood.

Firewood and outage preparedness usually go together on rural property. If your backup plan includes electric loads, size those separately with the generator sizing calculator.