How this is calculated
Dry wood is chemically almost the same everywhere: a pound of oven-dry oak and a pound of oven-dry pine both release close to 8,600 BTU. What differs is how many pounds fit inside a fixed stack, and that is density.
So the chart runs one line of arithmetic per species: specific gravity from the USDA Forest Products Laboratory Wood Handbook (FPL-GTR-190), times 62.4 lb per cubic foot for water, times the 80 cubic feet of solid wood in a stacked 128 cubic foot cord, times 8,600 BTU per pound of oven-dry wood.
Both constants are the extension convention rather than anything invented here. University of Maryland Extension FS926 defines its standard cord as 128 cubic feet of space holding 80 cubic feet of solid wood; University of Arkansas FSA1091 states the 8,600 BTU per pound.
White oak, specific gravity 0.60: 0.60 × 62.4 = 37.4 lb per cubic foot; × 80 = 2,995 lb of oven-dry wood in the cord; × 8,600 = 25.8 million BTU. Red oak at 0.56 gives 24.0 million; eastern white pine at 0.34 gives 14.6 million — same chemistry, half the wood.
The kWh per stere column uses the European convention, which is different accounting. American charts quote gross heat in bone-dry wood; European tables quote net calorific value at 20 percent moisture, subtracting the heat that leaves as water vapor up the flue.
The net values used here are the Bavarian TFZ measurements at 20 percent moisture — 14.3 MJ per kg for beech, 14.1 for oak, 14.6 for spruce, 14.9 for pine — applied to the same 0.625 packing factor as the cord column, so the two columns describe the same stack.
Where this lands against a published table
Extension tables are built from measured cord weights rather than from specific gravity, so they scatter around the same arithmetic. These are the million-BTU-per-cord figures University of Maryland Extension FS926 prints for a standard 80 cubic foot cord at 20 percent moisture, alongside the chart above.
| Species | Maryland FS926 | This chart |
|---|---|---|
| White ash | 23.6 | 23.6 |
| Eastern red cedar | 18.9 | 18.9 |
| Black walnut | 21.8 | 21.9 |
| Black locust | 28.1 | 28.3 |
| White oak | 27.0 | 25.8 |
| Shagbark hickory | 29.1 | 27.5 |
The gap on the dense species is cord weight. Maryland measured 3,863 lb of air-dried white oak per cord where green specific gravity predicts 3,744, and its hickory is heavier again.
Both sit inside a much larger uncertainty. FS926 puts the solid wood in a cord at 65 cubic feet for small crooked sticks rising to about 90 for large straight ones, with 80 as its regional average. That is roughly 15 percent either way, wider than any disagreement between species tables.
Steres carry the same problem in metric. TFZ’s own table prints 1,458 kWh per stere for beech at 20 percent, against 1,730 here, because TFZ stacks loose one-meter billets at about 0.50 cubic meters of solid wood per stere while this chart uses 0.625. Ask what a seller’s stere is stacked from before comparing two prices.
Working out your winter
A house loses heat in proportion to floor area, shell leakiness and winter severity. Heating degree days measure the last part, so annual heat need equals degree days × 24 hours × floor area × a shell factor of 0.07 to 0.30 BTU per hour per degree per square foot. Divide by heat per cord and appliance efficiency to get cords.
An 1,800 square foot house of average construction in a 5,500 degree-day climate needs roughly 47 million BTU a season — about 2.8 cords of red oak in a certified stove.
For scale, the EIA Residential Energy Consumption Survey (Table CE7.2, 2015 data, released December 2018) found that the 3.5 million American homes heating mainly with wood burned 88.8 million BTU each. At the 20 million BTU per cord the survey uses, that is about 4.4 cords, and 5.6 cords in the Northeast.
A 165 square meter house of average construction in a 3,000 degree-day climate needs roughly 13,500 kWh a season — about 2.7 cords of European oak in a certified stove, just under 10 steres. Eurostat’s nrg_chdd series counts only days whose mean temperature falls to 15°C or below, then sums 18°C minus that mean, so take your country’s figure from that series rather than converting an American 65°F number.
The second mode skips the modeling: tell it what you burned in oil, gas, electricity or pellets last winter and it converts that to delivered heat, then cords.
The moisture penalty
Moisture moves firewood heat further than species choice ever does. The figures below are the measured beech table published by the Technologie- und Förderzentrum in Straubing, Bavaria, dated October 2015. Moisture here is water as a share of total weight, the European convention.
| Moisture content | kWh per kg | kg per stere | kWh per stere | What it means |
|---|---|---|---|---|
| 0% (oven dry) | 5.11 | 343 | 1,757 | Laboratory reference; never occurs outdoors |
| 10% | 4.53 | 355 | 1,613 | Kiln dried, or three summers in a dry shed |
| 20% | 3.97 | 368 | 1,458 | The target on both sides of the Atlantic |
| 30% | 3.39 | 403 | 1,361 | Hisses, blackens the glass, starts laying creosote |
| 40% | 2.81 | 470 | 1,315 | One summer of seasoning on dense hardwood |
| 50% | 2.22 | 564 | 1,252 | Green, straight off the saw |
TFZ stacks loose one-meter billets, roughly 0.50 cubic meters of solid wood per stere, so its kWh per stere sits below the chart above. What matters in this table is the ratio down each column.
Read the last two columns against each other and the trap appears. Per kilogram, green wood carries less than half the heat, because a third of what you carried in was water. Per stere the loss from 20 percent to 50 percent is only about 14 percent, from 1,458 kWh to 1,252, since the same logs hold the same wood.
That 14 percent is not where the damage happens. A fire fed wet wood runs cool, so the smoke never reaches the temperature at which it burns, and unburned smoke is both the heat you paid for and the creosote now lining the chimney. Regulators write the rule as a moisture limit for that reason.
EPA Burn Wise puts the line at under 20 percent, tested with a moisture meter, and asks for at least six months of seasoning stored off the ground with only the top covered, since covering the sides traps moisture in.
In England the same limit is a sales rule. Under the Air Quality (Domestic Solid Fuels Standards) (England) Regulations 2020, wood sold in volumes under 2 cubic meters has had to be certified Ready to Burn at 20 percent moisture or less since 1 May 2021.
Cords, face cords and steres
Most firewood arguments are unit arguments. A full cord is a stack of 128 cubic feet, four feet high by four deep by eight long. A face cord shares that face but is only as deep as the pieces are long, so its size depends on the cut length.
Arkansas adopted the Uniform National Standards in 2001 and the rule there is typical of the states that did. Firewood may be sold only by the cord, a fraction of a cord, or the cubic meter, and the seller must hand over a delivery ticket or sales invoice naming the species, the quantity, the price and the vendor.
A price per truckload or per rick meets none of that, and cannot be compared with anything.
Across Europe wood is sold by the stere, also written Raummeter or Ster, or by the loose cubic meter of chips called the Schüttraummeter, which holds roughly 40 percent less wood. Ask which measure a price uses.
Reading the splitting, coaling and smoke columns
- Splitting. Elm, sweetgum and sycamore have interlocked fibers that defeat a maul and want a hydraulic splitter; oak, ash and honeylocust split clean off the round.
- Coaling. Dense oaks, hickory, hornbeam, holm oak and olive leave a coal bed that still holds fire in the morning; softwoods and poplars burn to ash, which is why an overnight burn is a hardwood job.
- Smoke and sparks. Resinous conifers and eastern red cedar throw sparks and suit a closed stove; sweet chestnut is famous for popping.
Seasoning time follows density: six to nine months for split softwood, six to twelve for ash and birch, twelve to eighteen for beech and maple, a full two years for dense oak and olive. Splitting small, stacking in single rows across the wind, keeping the pile off the ground and covering only its top roughly halves those figures.
Quick answers
Which firewood has the highest BTU per cord?
Osage orange and live oak lead, at 31 to 34 million BTU per cord depending on whose cord weights you use, matched by holm oak and olive in southern Europe. Black locust, hickory and hornbeam follow at 27 to 29 million.
How many cords of wood do I need for a winter?
About four and a half. EIA's 2015 Residential Energy Consumption Survey put the average American wood-heated home at 88.8 million BTU a year, and counts a cord as 20 million BTU. The real range runs from under two to over eight.
How long does firewood take to season?
Six to nine months for split softwood, six to twelve for ash and birch, twelve to eighteen for beech and maple, up to twenty-four for dense oak and olive. EPA Burn Wise treats six months as the floor.
How much heat does wet firewood really lose?
Per kilogram, beech falls from 5.11 kWh bone dry to 2.22 kWh at 50 percent moisture. Per stere the loss between 20 and 50 percent is nearer 14 percent, but the cool fire takes efficiency and chimney safety with it.
Is softwood bad for a wood stove?
No. Seasoned spruce and pine burn cleanly; they simply carry less heat per cord, so you handle about twice the volume.
Sources
- Specific gravity. USDA Forest Products Laboratory, Wood Handbook, FPL-GTR-190, green-volume basis. European species from the published European wood-density literature, which carries a wider spread than the North American values.
- The 80 cubic foot cord, and the FS926 comparison table. University of Maryland Extension FS926, Heating with Wood.
- 8,600 BTU per pound, the burn ratings and the Arkansas sale rule. University of Arkansas FSA1091, Basics of Heating with Firewood, revised July 2019, reprinting USDA Forest Service Leaflet 559.
- Moisture and calorific values. Technologie- und Förderzentrum Straubing, Heizwerttabellen für verschiedene Holzarten, TFZ-Merkblatt 15BKu005, October 2015.
- Moisture limits. EPA Burn Wise best practices; the Air Quality (Domestic Solid Fuels Standards) (England) Regulations 2020, in force 1 May 2021, and the Ready to Burn certification that enforces them.
- Household wood use. US Energy Information Administration, Residential Energy Consumption Survey, Table CE7.2, 2015 data released December 2018. Fuel energy contents also from EIA.
- Degree days. Eurostat nrg_chdd: days counted below a 15°C mean, summed against an 18°C reference.
Every figure on this page was re-checked against these sources in September 2026.
Go deeper
Wood is one column of the winter energy budget. Run the whole house through the off-grid house calculator, size the appliance with the wood stove sizing guide, and for mass rather than a stove see the rocket mass heater build. In Europe, check the Ecodesign 2022 stove rules. More under heating and cooling.