BTU (British thermal unit)
A BTU is the energy needed to raise one pound of water 1 °F; stove heat output is rated in BTU per hour. Certified stoves publish an output range — roughly 10,000–20,000 BTU/h at low burn up to 30,000–80,000 BTU/h at peak, with about 25–35 BTU/h per square foot as a sizing rule of thumb.
The range matters more than the peak: a stove spends most of its life at low-to-medium output, and the low end determines whether it can idle in mild weather without cooking the room. The EPA database records the output range measured during certification testing.
For fuel comparisons, a cord of seasoned hardwood holds about 22 million BTU (MMBtu) and a ton of wood pellets about 16.4 MMBtu — before multiplying by the appliance’s efficiency.
| Unit conversions | 1 BTU ≈ 1,055 joules; 1 kWh = 3,412 BTU; 1 therm = 100,000 BTU |
|---|---|
| Sizing rule of thumb | 25–35 BTU/h per square foot at design temperature |
| Certified low ends | Median around 13,000 BTU/h across current room heaters |
| Cord of seasoned hardwood | About 22 MMBtu (oak and hickory near 24; pine 15–17) |
| Ton of wood pellets | About 16.4 MMBtu, roughly 8,200 BTU/lb |
| Other fuels | 91,500 BTU/gal propane; 138,500 BTU/gal #2 fuel oil; 100,000 BTU/therm natural gas |
Input BTU and output BTU are different numbers
Gas and oil appliances are conventionally rated on fuel input: an “80,000 BTU” gas furnace burns that much fuel per hour and, at 80% efficiency, delivers about 64,000 BTU/h into the house. The EPA certifies wood heaters the other way round, on measured heat output — the number in the database is already net of the appliance’s efficiency.
That makes cross-fuel comparison a trap unless you convert. A 40,000 BTU/h certified wood stove is not half the heater that an 80,000 BTU/h gas furnace is; it is roughly two-thirds of it. And a stove brochure quoting a much larger “BTU” figure than the EPA record is usually publishing a theoretical fuel-input number computed from firebox size, which nobody measured.
Use the certified output range for anything that matters. It is the only figure on the sheet produced by burning the appliance under a defined protocol, and it comes with both a top and a bottom.
The range matters more than the peak
A house needs its design heat load — perhaps 25–35 BTU/h per square foot — only on the coldest days of the year. Most of the heating season is spent at a third to a half of that. A stove chosen on peak output alone will therefore be run at its minimum for the majority of its life, and the minimum is where clean combustion is hardest.
Median turndown across the 158 currently certified room heaters is about 2.6:1, so a stove that peaks at 45,000 BTU/h typically bottoms out near 17,000. In a well-insulated 1,200 sq ft house that lower figure is still too much heat on a 40 °F evening, and the owner ends up either opening windows or closing the air control past the point where the stove burns cleanly.
So check the low end against a mild-weather load, not just the high end against a design-day load. The certified range on each model page gives you both, and the sizing calculator turns square footage, insulation, and climate into a target band to check them against.
Doing the fuel arithmetic
Cost per delivered million BTU is the only fair way to compare heating fuels, and it takes one line: price per unit ÷ MMBtu per unit ÷ efficiency. Cord wood at $250 a cord and 22 MMBtu, burned in a 75% stove, is $250 ÷ 22 ÷ 0.75 ≈ $15 per delivered MMBtu. Pellets at $300 a ton and 16.4 MMBtu in an 80% stove are $300 ÷ 16.4 ÷ 0.80 ≈ $23. Electric resistance heat at $0.16/kWh is $0.16 × 293 ≈ $47, since a million BTU is 293 kWh.
Those figures move a lot with local prices, and they ignore your own labour, so cheap or self-cut wood swings the comparison much further than any difference in appliance efficiency does. The wood heat cost calculator runs the same arithmetic against current prices for wood, pellets, gas, oil, propane, and electricity.
The one number to be careful with is the cord. A cord is 128 cubic feet stacked, not a pickup load and not a face cord — and the energy content varies by roughly 50% between dense hardwood and softwood, because a cord of pine simply contains less wood. Price per cord means nothing until you know the species and that the wood is genuinely under 20% moisture.
Where the sizing rule breaks
The 25–35 BTU/h per square foot band assumes a reasonably insulated, reasonably tight house in a mixed climate with normal ceiling heights. It is a starting point that is wrong in both directions often enough to be worth checking.
It runs low for an uninsulated or single-glazed older house, a cathedral ceiling, a walkout basement, or a genuinely cold climate — 45–50 BTU/h per square foot is realistic for a draughty farmhouse in a northern winter. It runs high for a modern tight build with good windows, where 15 BTU/h per square foot may cover it, and it is meaningless for a passive-standard house that needs almost nothing.
Floor plan matters as much as area, because a stove heats by radiation and convection from one spot. Two thousand square feet in an open plan around a central stove is a comfortable job; the same area split into small rooms down a corridor is not, no matter how many BTUs the stove makes. That is a distribution problem, and the answer is a furnace, a boiler, or a second appliance rather than a bigger stove.
Frequently asked questions
How many BTU do I need to heat 1,000 square feet?
Roughly 25,000–35,000 BTU/h at design temperature for a reasonably insulated house in a mixed climate. Adjust up toward 45,000–50,000 for a poorly insulated or very cold-climate building, and down toward 15,000–20,000 for a tight modern build. Just as important, check that the stove’s certified low output is small enough for a mild evening in that space.
How many BTU are in a cord of wood?
About 22 million BTU for a cord of seasoned mixed hardwood. Dense species run higher — oak and hickory approach 24 MMBtu — and softwoods lower, at roughly 15–17 MMBtu for pine, because a cord of pine contains less wood by weight. Multiply by the appliance efficiency to get delivered heat: 22 MMBtu at 75% is about 16.5 MMBtu into the house.
Is a higher-BTU wood stove better?
No — matching matters more than maximum. An oversized stove has to be damped down to keep the room bearable, which is exactly the condition where combustion goes dirty, glass blackens, and creosote forms. Check the low end of the certified range against a mild-weather load as carefully as you check the peak against a design day.
What is the difference between BTU input and BTU output?
Input is the fuel energy an appliance consumes per hour; output is the heat it actually delivers. Gas and oil equipment is usually rated on input, wood heaters on measured output. So an 80,000 BTU/h input gas furnace at 80% efficiency delivers about 64,000 BTU/h — the number to compare against a wood stove’s certified output figure.
How many BTUs is a ton of wood pellets?
About 16.4 million BTU, or roughly 8,200 BTU per pound — a ton is fifty 40 lb bags. Softwood pellets typically run slightly higher than hardwood. At 80% appliance efficiency that ton delivers about 13 MMBtu, which is comparable to roughly two-thirds of a cord of hardwood burned in a stove of similar efficiency.