Understanding your result
The headline is the heat rate in kilowatts, the unit the rest of the world uses for heating and cooling capacity, with the BTU per hour, watts and tons in the sub-line. The stats repeat BTU/hr, watts and tons, the three figures most likely to be needed when reading a US nameplate alongside a European or Asian one.
The table lists all eight rate units and ends with the energy over the hours you entered, in BTU, kilowatt-hours and therms. That last row is the bridge between a rating and a bill: a 100,000 BTU/hr furnace that fires for 200 hours in a month has used 20 million BTU, which is 200 therms of gas input or would be 5,862 kWh if the heat came from electric resistance.
For air conditioner capacities specifically, the AC tonnage converter adds the Asian horsepower class and the half-ton stock sizes. For turning energy into money, the heating cost by fuel calculator prices a season’s heat for each fuel and the fuel cost per million BTU calculator compares fuels per unit of delivered heat. To convert watts of equipment in a room into cooling load, the equipment heat load calculator applies the same 3.412 factor with the people and lighting added.
How we calculate this
Every input is converted to BTU per hour first, then to each other unit, so no rounding carries from one line to the next. The factors are definitions or standard equivalents: 1 kWh = 3,412.14 BTU (1 BTU = 1,055.06 J), 1 ton = 12,000 BTU/hr, 1 therm = 100,000 BTU, 1 boiler horsepower = 33,475 BTU/hr, 1 BTU = 0.252 kcal. They are collected with the airflow, pressure and flow conversions on the unit conversions page, and the fuel energy contents that turn therms and gallons into BTU are on the fuel energy content page.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| BTU per kWh | 3,412.14 | 1 BTU = 1,055.06 J (International Table BTU); EIA uses 3,412 |
| BTU/hr per ton | 12,000 | Definition of the ton of refrigeration |
| BTU per therm | 100,000 | Definition; EIA |
| BTU/hr per boiler horsepower | 33,475 | ASME definition (34.5 lb of steam per hour from and at 212 °F) |
| kcal per BTU | 0.252 | Definition (1 kcal = 4,184 J) |
| MBH | 1,000 BTU/hr | Industry convention (M = thousand) |
Assumptions last reviewed October 7, 2026.
The converter changes units and nothing else. It does not apply efficiencies (a furnace’s input rate is not its output rate), it does not know a heat pump’s COP, and the energy row assumes the rate is constant over the hours entered, which a cycling appliance’s is not. The guide to furnace input, output and AFUE explains the efficiency step, and the guide to SEER2, HSPF2, AFUE and COP covers how a rating turns a heat rate into an electricity draw.
Two worked examples
A 100,000 BTU/hr furnace
The input rating of a common furnace, run for one hour.
- Kilowatts: 100,000 ÷ 3,412.14 = 29.3 kW (29,307 W)
- Tons: 100,000 ÷ 12,000 = 8.33 tons
- MBH: 100 MBH; therms per hour: 1.000; boiler hp: 2.99; kcal/h: 25,200
- Energy over 1 hour: 100,000 BTU = 29.31 kWh = 1 therm
One hour of firing burns exactly one therm of gas, before efficiency; at 95% AFUE the house receives 95,000 BTU of it. If the same 100,000 BTU/hr were delivered by electric resistance, the meter would turn 29.3 kWh in that hour, which at 17 cents is $4.98 against $1.50 for the therm.
A 3.5-ton heat pump running for 8 hours
The nominal capacity of a 3.5-ton unit, treated as a steady heat rate for an 8-hour night.
- BTU/hr: 3.5 × 12,000 = 42,000 BTU/hr
- Kilowatts: 42,000 ÷ 3,412.14 = 12.3 kW (12,309 W) of heat
- Boiler hp: 1.25; kcal/h: 10,584; therms per hour: 0.42
- Energy over 8 hours: 336,000 BTU = 98.47 kWh = 3.36 therms
The 98.47 kWh is heat delivered, not electricity used; a heat pump with a COP of 3 would draw about 33 kWh to deliver it, and a 95% gas furnace would burn 3.54 therms. The converter shows the energy; the heating cost by fuel calculator divides by the efficiencies and multiplies by the prices.
Where to find your inputs
Furnace and boiler ratings. On the nameplate inside the cabinet, as “INPUT” and “OUTPUT” or “CAPACITY” in BTU/hr; MBH on commercial equipment.
Air conditioner and heat pump capacities. In the model number in thousands of BTU/hr (024, 036, 048) and on the outdoor unit’s nameplate; in kW on equipment sold outside North America.
Watts. On the label of any electric heater or appliance; the figure is both its electricity draw and, for resistance heat, its heat output.
Hours. From a run-time meter, a smart thermostat’s history, or an estimate of firing hours; divide a month’s therms by the input in therms per hour to find the hours a gas furnace fired.
Common mistakes
- Treating BTU as BTU per hour. BTU is energy, BTU/hr is a rate; a 100,000 BTU furnace is a 100,000 BTU per hour furnace.
- Reading MBH as millions. M is a thousand; MMBtu is a million.
- Confusing boiler horsepower with mechanical horsepower. 33,475 against 2,544 BTU/hr.
- Skipping the efficiency. Input ratings are fuel burned; multiply by AFUE for heat delivered.
- Assuming the rate is constant. Equipment cycles; the energy row is for the hours the burner or compressor actually ran.
- Mixing cooling capacity and electrical draw in kW. Capacity is heat moved; divide by COP or EER to get electricity.
Questions people ask
- How many watts is a BTU per hour?
- 0.293 watts. One BTU is the heat that raises a pound of water by one degree Fahrenheit, about 1,055 joules, so one BTU per hour is 1,055 J ÷ 3,600 s = 0.293 W. Going the other way, one watt is 3.412 BTU per hour and one kilowatt is 3,412 BTU per hour. A 1,500 W space heater therefore delivers 5,118 BTU per hour, and a 100,000 BTU per hour furnace delivers 29.3 kW.
- What is MBH?
- Thousands of BTU per hour, from the Roman numeral M for a thousand. Boilers, furnaces and commercial equipment are often rated in MBH, so a 100 MBH boiler is 100,000 BTU per hour. It is not a million; the million is written MMBH or, for energy, MMBtu, which is how fuel is priced per million BTU.
- How do I convert a furnace rating to therms?
- A therm is 100,000 BTU of energy, so a 100,000 BTU per hour furnace burns one therm of gas per hour of firing at full input. Multiply by the hours it runs, so 300 hours of firing in a month is 300 therms, before any correction for efficiency, which is already inside the input rating. Divide a gas bill in therms by the furnace's input in therms per hour to find how many hours it fired.
- What is boiler horsepower?
- A commercial boiler unit equal to 33,475 BTU per hour, the heat needed to evaporate 34.5 pounds of water per hour from and at 212 °F. It has nothing to do with the 2,544 BTU per hour of a mechanical horsepower or with the air-conditioner "horsepower" class used in Asia. A 10 boiler horsepower unit is about 335,000 BTU per hour or 98 kW.
- How do tons relate to BTU and kW?
- One ton of refrigeration is 12,000 BTU per hour, the cooling from melting a short ton of ice in 24 hours, and equals 3.517 kW of cooling. The converter gives tons for any heat rate, so a 100,000 BTU per hour heating load is 8.33 tons, which is a useful comparison when a heat pump is being considered for a house whose heating load is known in BTU.
- Why does the energy row say kWh and therms for the same number?
- Because a quantity of heat can be expressed in any energy unit. 100,000 BTU is 29.31 kWh and exactly one therm. If that heat came from electric resistance it would cost 29.31 kWh of electricity; from a 95% gas furnace it would cost 1 ÷ 0.95 = 1.05 therms. The converter shows the energy itself; the heating cost by fuel calculator applies the efficiencies and prices.