BTU Calculator
Calculate heat energy in BTU and convert between BTU, kWh, joules, kJ, MJ, and MMBtu. You can also estimate heat required from mass, specific heat, and temperature change.
Do not mix an amount of heat energy with a rate of heat transfer. For example, 10,000 Btu is energy; 10,000 Btu/hr is thermal power.
Calculation Results
BTU Conversion Reference Table
| Unit | Equivalent in Btu |
|---|---|
| 1 J | ≈ 0.000947817 Btu |
| 1 kJ | ≈ 0.947817 Btu |
| 1 MJ | ≈ 947.817 Btu |
| 1 Wh | ≈ 3.412142 Btu |
| 1 kWh | ≈ 3,412.142 Btu |
| 1 therm | = 100,000 Btu |
| 1 MMBtu | = 1,000,000 Btu |
Values are derived from a consistent BtuIT = 1,055.056 J basis. [Source: NIST]
MMBtu Reference
MMBtu means one million Btu. The notation is commonly used in natural-gas, boiler, utility, and industrial-energy contexts.
MMBtu is an amount of energy. MMBtu/hr is an energy-transfer rate. Do not interpret the two “M” characters as multiplying one million by another one thousand; MMBtu is conventionally used to mean one million Btu.
Therm Reference
A therm is an energy unit commonly used in natural-gas billing and energy accounting.
Therm is an amount of energy, not a heat-transfer rate. Do not treat therm and Btu/hr as interchangeable quantities.
BTU and HVAC
HVAC equipment is commonly rated by a heat-transfer rate rather than total heat energy. Heating and cooling capacities therefore appear in Btu/hr, kBtu/hr, tons of refrigeration, or kW. A 12,000 Btu/hr air conditioner does not mean it stores 12,000 Btu; it describes a nominal cooling rate.
This is a conventional HVAC capacity conversion. Actual equipment sizing also depends on building load, climate, envelope, ventilation, occupancy, internal gains, and equipment performance.
BTU and Boiler / Steam Systems
Btu is widely used in boilers, steam systems, furnaces, heat exchangers, process heating, natural gas, and industrial energy systems. Boiler output or input may be expressed in Btu/hr, MMBtu/hr, boiler horsepower, or steam flow depending on the engineering task.
- Boiler heat input and useful output
- Steam and condensate energy balances
- Furnace and process-heating duty
- Heat-exchanger load
- Natural-gas consumption and energy accounting
Related tools: Boiler Efficiency Calculator, Boiler Horsepower Calculator, Boiler Blowdown Calculator, and Steam Flow Calculator.
What Is a BTU?
A British thermal unit (Btu) is a unit of heat energy. DOE describes the Btu as a quantity of heat associated with raising the temperature of one pound of water by one degree Fahrenheit under specified reference conditions. Historical definitions use different reference conditions, which is why a precise engineering conversion should state which Btu convention is being used.
This calculator uses the International Table Btu for general engineering conversions rather than mixing multiple historical definitions.
BTU Conversion
NIST lists the International Table British thermal unit conversion as:
Once a Btu value is converted to joules, it can be converted consistently to kJ, MJ, Wh, kWh, or other supported energy units. This calculator uses a single internal energy basis to avoid path-dependent rounding differences.
BTU to kWh
Because 1 kWh equals 3.6 million joules and one BtuIT equals 1,055.056 joules, the conversion can be written as:
For example, 10,000 Btu is approximately 2.9307 kWh. This is an energy conversion; it does not by itself describe how quickly the energy is delivered.
kWh to BTU
To convert electrical or thermal energy expressed in kWh to Btu, multiply by approximately 3,412.142:
For example, 5 kWh is approximately 17,060.71 Btu on the BtuIT basis used by this calculator.
BTU vs BTU/hr
Btu is energy. Btu/hr is the rate at which energy is transferred. These quantities are related in the same way that kWh is related to kW: one describes an amount, while the other describes a rate.
Always identify whether an HVAC, boiler, heater, or cooling-system number is an energy amount or a capacity/rate before converting units.
How to Calculate Heat Energy
For a simplified sensible-heat calculation with no phase change, heat energy can be estimated from mass, specific heat, and temperature change:
Here, Q is heat energy, m is mass, c is specific heat capacity, and ΔT is final temperature minus initial temperature. The specific heat must use units consistent with the mass and temperature interval. Real engineering calculations may require temperature-dependent properties, phase-change enthalpy, pressure effects, and heat losses.
What Is MMBtu?
MMBtu is a commonly used notation for one million British thermal units. It appears frequently in natural-gas pricing, industrial energy reporting, boilers, and process heating.
The “/hr” changes the meaning: MMBtu is energy, while MMBtu/hr is a thermal rate.
BTU in HVAC
HVAC heating and cooling capacity is usually expressed as a rate, commonly Btu/hr rather than Btu. For example, a nominal 12,000 Btu/hr cooling capacity corresponds to one refrigeration ton. Larger equipment may use kBtu/hr, tons, or kW.
BTU capacity alone is not enough to size HVAC equipment. A real load calculation must account for building envelope, outdoor design conditions, ventilation, infiltration, occupancy, internal loads, humidity, and equipment performance.
BTU in Boilers and Industrial Systems
In industrial thermal systems, Btu-based units may represent fuel input, useful thermal output, process duty, or total energy consumption. Larger values are often expressed as MMBtu/hr, MW, steam flow, or boiler horsepower depending on the application.
Always confirm the calculation boundary. Fuel input in MMBtu/hr is not the same as useful boiler output unless efficiency is accounted for, just as steam mass flow is not itself an energy rate without the applicable enthalpy change.
BTU vs Joule
| Unit | System / Meaning |
|---|---|
| Btu | U.S. customary / British thermal energy unit used widely in HVAC and energy industries. |
| Joule (J) | SI energy unit used in scientific and engineering calculations. |
BTU vs Therm
Both Btu and therm are energy units. The therm is much larger and is frequently used for natural-gas energy billing.
Neither unit alone describes a rate. A time denominator such as “per hour” is required to describe thermal power.
Choosing the Correct Unit
- Heating / cooling capacity: Btu/hr, kBtu/hr, kW, refrigeration tons.
- Energy consumption: Btu, kWh, kJ, MJ.
- Natural gas: therm, MMBtu.
- Industrial boiler duty: Btu/hr, MMBtu/hr, MW thermal, steam flow.
- Scientific engineering: J, kJ, MJ, W, kW.
Select a unit that matches the physical quantity first—energy or energy rate—then perform the numerical conversion.
2026 Reference Note
The fundamental BTU conversion factors used by this calculator are established unit relationships and are not a new 2026 regulatory value. For current reference use, this calculator applies the NIST International Table Btu relationship:
It also uses the SI relationship 1 kWh = 3,600,000 J, giving approximately 3,412.142 Btu per kWh and 0.293071 W per Btu/hr.
Which BTU Does This Calculator Use?
Historical and industry references can define Btu using different conventions, including BtuIT, Btuth, mean Btu, and reference-temperature definitions. Those values are close but not identical.
For general engineering conversions, this calculator uses the International Table Btu listed by NIST:
This keeps energy conversions internally consistent and avoids mixing different historical Btu definitions in the same calculation.
Limitations
This calculator performs unit conversions and simplified sensible-heat calculations. Actual HVAC, boiler, heat-exchanger, process-heating, and equipment-sizing calculations may require equipment specifications, fluid properties, temperature-dependent specific heat, pressure, efficiency, heat losses, environmental conditions, manufacturer data, and applicable engineering standards.
Do not use a simple BTU conversion as a complete HVAC load calculation, boiler selection, or process-equipment design.
Engineering References
Primary conversion reference for the International Table Btu and SI energy units, including BtuIT to joule and kWh to joule relationships.
NIST Guide to the SI, Appendix B.9Reference for the British thermal unit as a heat-energy unit and related energy terminology.
DOE Full Text GlossarySupplementary DOE glossary context for Btu as a standard heat-energy measurement used to compare energy sources.
DOE Energy Data Management Guide GlossaryFrequently Asked Questions
1. What does BTU stand for?
BTU stands for British thermal unit. It is a unit of heat energy used widely in HVAC, natural gas, boilers, and general energy work. Precise historical Btu definitions can differ slightly because they use different water-temperature reference conditions. For consistent engineering conversions, this calculator uses the International Table Btu value listed by NIST.
2. How many kWh is 1 BTU?
Using the International Table Btu, one Btu is approximately 0.000293071 kWh. The conversion comes from 1 Btu = 1,055.056 J and 1 kWh = 3,600,000 J. Because Btu and kWh are both energy units, they can be converted directly without a time factor.
3. How many BTU are in 1 kWh?
One kWh is approximately 3,412.142 Btu on the International Table Btu basis. This is an energy conversion. If the value is kW rather than kWh, then you are working with power, and the corresponding thermal-rate conversion is approximately 3,412.142 Btu/hr per kW.
4. How many joules are in a BTU?
This calculator uses 1 International Table Btu = 1,055.056 joules, the factor listed by NIST. You may see slightly different numbers in older engineering references because different Btu definitions have been used historically. For consistent results, avoid mixing Btu conventions within the same calculation.
5. Is BTU a unit of energy or power?
Btu is a unit of energy. It describes an amount of heat. Power requires a time basis, such as Btu/hr. The distinction matters in HVAC and boilers because a heating system may be rated at 100,000 Btu/hr, while its energy consumption over a period is measured in Btu, kWh, therms, or MMBtu.
6. What is the difference between BTU and BTU/hr?
Btu measures an amount of energy, while Btu/hr measures how quickly energy is transferred. A quantity of 10,000 Btu could be delivered over minutes or hours. A rate of 10,000 Btu/hr means the system transfers energy at that rate while it is operating. They should not be treated as interchangeable units.
7. How many BTU are in 1 MMBtu?
One MMBtu equals exactly 1,000,000 Btu by the convention used in energy industries. MMBtu is an energy quantity. MMBtu/hr is different because it represents an energy rate. One MMBtu is approximately 293.071 kWh on the BtuIT conversion basis used by this calculator.
8. How many BTU are in a therm?
One therm equals 100,000 Btu. Therms are commonly used for natural-gas billing and energy reporting. Like Btu, a therm is an amount of energy rather than a power unit. If you need a heating rate, use Btu/hr, kW, or another unit that includes time.
9. How do I calculate BTU from temperature change?
For a simplified sensible-heat calculation with no phase change, use Q = m × c × ΔT. The mass, specific heat, and temperature interval must use compatible units. Water is often approximated near 1 Btu/lb·°F in simple examples, but actual specific heat varies with temperature and material condition, so verified properties may be needed.
10. How many BTU/hr is 1 kW?
One kW is approximately 3,412.142 Btu/hr. This is a power conversion derived from the same energy relationships used for kWh and Btu. Because both kW and Btu/hr are rates, no additional operating-time assumption is needed to convert between them.
11. How many BTU/hr is 1 ton of cooling?
One refrigeration ton is conventionally equal to 12,000 Btu/hr of cooling capacity. This is an HVAC capacity conversion, not a total amount of stored energy. Actual air-conditioning equipment selection also depends on sensible and latent load, airflow, climate, building envelope, occupancy, ventilation, and equipment performance.
12. Why do different sources sometimes show slightly different BTU conversion values?
Several historical Btu definitions exist, including the International Table Btu, thermochemical Btu, mean Btu, and definitions at specified water temperatures. Their values are close but not identical. This calculator consistently uses BtuIT = 1,055.056 J so that all conversions follow one stated engineering basis.
13. Can this calculator be used for HVAC sizing?
It can convert HVAC energy and capacity units and perform a simplified heat calculation, but it is not a complete HVAC load-sizing tool. Equipment sizing normally requires design temperatures, building envelope, solar gain, infiltration, ventilation, humidity, occupancy, internal gains, duct effects, and manufacturer performance data.
14. Can BTU be converted to kW?
Btu by itself is energy, while kW is power, so a time interval is required to convert an amount of Btu into kW. If the source quantity is Btu/hr, then it can be converted directly to kW: 1 Btu/hr is approximately 0.000293071 kW. Use the Heating / Cooling Rate mode when time is part of the problem.
15. What BTU definition does this calculator use?
The calculator uses the International Table British thermal unit, BtuIT, with 1 Btu = 1,055.056 J. This NIST-listed value provides a consistent basis for converting to joules, kWh, MJ, and related units. Other historical Btu definitions are not mixed into the calculator.
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Engineering Disclaimer
This calculator provides unit conversions and simplified heat-energy calculations for informational and preliminary engineering use. Actual HVAC, boiler, process heating, heat exchanger, and equipment sizing calculations may require verified material properties, operating conditions, manufacturer specifications, applicable engineering standards, and qualified professional review.