What is British thermal unit?
A British thermal unit (BTU) is an energy unit defined as the heat needed to raise one pound of water by one degree Fahrenheit. It is used in HVAC systems, heating and cooling equipment ratings, and natural gas billing in the United States.
Real-world uses
The British Thermal Unit is used in the United States for heating and cooling system ratings (furnaces, air conditioners), natural gas billing, and refrigeration. A typical home air conditioner might be rated at 12,000–24,000 BTU/h. Natural gas is often sold per therm (100,000 BTU).
History
The British Thermal Unit was proposed in the 1840s as the heat needed to raise one pound of water by one degree Fahrenheit. Despite its name, its primary use today is in the United States; the UK and most Commonwealth countries have transitioned to SI units for engineering applications.
Common mistakes
Confusing BTU (energy) with BTU/h (power). An air conditioner rated at "12,000 BTU" typically means 12,000 BTU per hour of heat removal capacity, not a fixed energy amount. Also mixing up the slightly different BTU definitions (IT, thermochemical, 39°F, 60°F).
What is kilojoule?
A kilojoule is a unit of energy equal to 1,000 joules. It is commonly used in nutrition to express the energy content of food, as well as in thermodynamics and mechanical engineering.
Real-world uses
Kilojoules are the standard energy unit for food nutrition labelling in Australia, New Zealand, and parts of Europe (alongside kilocalories). A typical adult daily intake is about 8,700 kJ. They are also used in chemistry for reaction enthalpies and combustion energies.
History
The kilojoule became important in nutrition labelling as countries adopted SI-based energy reporting. Australia was among the first to mandate kJ on food labels in the 1970s. The EU requires both kJ and kcal on nutrition labels.
Common mistakes
Confusing kilojoules with kilocalories—1 kcal = 4.184 kJ. Food labels showing "2000 kJ" are about 478 kcal, not 2000 kcal. Also, some people incorrectly assume kJ and kW are the same thing; one is energy, the other is power.
When is this conversion used?
Converting between British thermal unit and kilojoule is common when working across metric and imperial systems, such as international trade, travel between countries with different measurement standards, or following instructions from a different region. Nutritional information is labelled in kilocalories in some countries and kilojoules in others, making this conversion important for dietary planning and food science.
Worked examples
1 British thermal unit = 1.05505585 kilojoule
1 kilojoule = 1,000 joule
How to convert British thermal unit to kilojoule
To convert British thermal unit to kilojoule, multiply the value by 1.05505585.
To convert kilojoule back to British thermal unit, multiply by 0.94781712.
Measurement standards
The joule is the SI derived unit of energy, defined as one kilogram-metre-squared per second-squared (kg·m²/s²). The kilowatt-hour, equal to exactly 3.6 megajoules, is accepted for use with the SI by the BIPM for practical electricity metering.
Did you know?
A single bolt of lightning releases roughly 1 to 5 gigajoules of energy, yet only a small fraction reaches the ground as electrical current. Most of the energy dissipates as light, heat, and the thunder shockwave.
Quick reference: British thermal unit to kilojoule
| British thermal unit | kilojoule |
|---|---|
| 0.1 | 0.10550559 |
| 0.5 | 0.52752793 |
| 1 | 1.05505585 |
| 2 | 2.11011171 |
| 5 | 5.27527926 |
| 10 | 10.55055853 |
| 25 | 26.37639632 |
| 50 | 52.75279263 |
| 100 | 105.50558526 |
| 250 | 263.76396315 |
| 500 | 527.52792631 |
| 1,000 | 1,055.05585262 |
Common values
| British thermal unit | kilojoule | |
|---|---|---|
| A food calorie (1 kcal) | 3.96566683 British thermal unit | 4.184 kilojoule |
| A AA battery | 8.87156825 British thermal unit | 9.36 kilojoule |
| Boiling a kettle | 398.08319053 British thermal unit | 420 kilojoule |
| Daily human diet (2000 kcal) | 7,931.33366278 British thermal unit | 8,368 kilojoule |
| 1 gallon of gasoline | 124,884.38377248 British thermal unit | 131,760 kilojoule |
Available Energy units
More British thermal unit conversions
- Convert British thermal unit to joule
- Convert British thermal unit to kilojoule
- Convert British thermal unit to kilowatt hour
- Convert British thermal unit to calorie (thermochemical)
- Convert British thermal unit to kilocalorie (dietary)
- Convert British thermal unit to watt hour
- Convert British thermal unit to megajoule
- Convert British thermal unit to electronvolt
- Convert British thermal unit to therm (US)
Assumption: calorie means thermochemical calorie, and therm means US therm.